feat(Board): add CandidateCard mini switcher

Reviewed-on: #50

.dockerignore

@@ -0,0 +1,9 @@
+node_modules
+.yarn/cache
+.yarn/unplugged
+.yarn/install-state.gz
+dist
+.git
+.gitea
+.svelte-kit
+storybook-static

.gitea/workflows/workflow.yml

@@ -41,4 +41,57 @@ jobs:
         run: yarn lint
 
       - name: Type Check
-        run: yarn check:shadcn-excluded
+        run: yarn check
+
+      - name: Run Unit Tests
+        run: yarn test:unit
+
+      - name: Run Component Tests
+        timeout-minutes: 5
+        run: yarn test:component --reporter=verbose --logHeapUsage
+
+  e2e:
+    needs: build
+    runs-on: ubuntu-latest
+    container:
+      image: mcr.microsoft.com/playwright:v1.59.0-jammy
+    steps:
+      - uses: actions/checkout@v4
+      - name: Enable Corepack
+        run: |
+          corepack enable
+          corepack prepare yarn@stable --activate
+      - name: Persistent Yarn Cache
+        uses: actions/cache@v4
+        with:
+          path: .yarn/cache
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: ${{ runner.os }}-yarn-
+      - name: Install dependencies
+        run: yarn install --immutable
+      - name: Build Svelte SPA
+        run: yarn build
+      - name: E2E Tests
+        timeout-minutes: 15
+        run: yarn test:e2e
+
+  publish:
+    # Runs if lint, unit-, component-, e2e-tests pass
+    needs: [build, e2e]
+    runs-on: ubuntu-latest
+    if: github.ref == 'refs/heads/main' # Only deploy from main branch
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Login to Gitea Registry
+      run: echo "${{ secrets.CI_DEPLOY_TOKEN }}" | docker login git.allmy.work -u ${{ gitea.repository_owner }} --password-stdin
+
+    - name: Build and Push Docker Image
+      run: |
+        docker build \
+          -t git.allmy.work/${{ gitea.repository }}:latest \
+          -t git.allmy.work/${{ gitea.repository }}:${{ gitea.sha }} \
+          .
+        docker push git.allmy.work/${{ gitea.repository }}:latest
+        docker push git.allmy.work/${{ gitea.repository }}:${{ gitea.sha }}

.gitignore

@@ -10,6 +10,12 @@ node_modules
 /build
 /dist
 
+# IDE settings
+.vscode
+
+# Git worktrees (isolated development branches)
+.worktrees
+
 # OS
 .DS_Store
 Thumbs.db
@@ -43,3 +49,7 @@ storybook-static
 
 # Tests
 coverage/
+.aider*
+playwright-report/
+blob-report/
+.playwright/

.oxlintrc.json

@@ -0,0 +1,195 @@
+{
+  "$schema": "./node_modules/oxlint/configuration_schema.json",
+  "plugins": ["import"],
+  "categories": {
+    "correctness": "error",
+    "suspicious": "warn",
+    "perf": "warn",
+    // style/restriction off: opt-in, contradictory grab-bags. Wanted rules enabled individually below.
+    "style": "off",
+    "restriction": "off"
+  },
+  "env": {
+    "browser": true,
+    "es2021": true
+  },
+  "ignorePatterns": [
+    "node_modules",
+    "dist",
+    "build",
+    ".svelte-kit",
+    ".vercel",
+    "*.config.js",
+    "*.config.ts"
+  ],
+  "rules": {
+    "no-console": "warn",
+    "no-debugger": "error",
+    "no-alert": "warn",
+
+    // no-cycle resolves $-aliases via tsconfig auto-discovery (no resolver config in oxlint)
+    "import/no-cycle": "error",
+    "import/no-duplicates": "warn",
+    "import/no-unassigned-import": "off", // CSS/side-effect imports are intentional
+
+    "no-sequences": "error",
+    "no-underscore-dangle": "off",
+    "no-shadow": "warn",
+    "no-implicit-coercion": "warn",
+    "no-await-in-loop": "warn",
+    "no-return-assign": "warn",
+    "no-new": "warn",
+    "no-unneeded-ternary": "warn"
+  },
+  // FSD boundaries. oxlint has no zone rule, so layer/segment direction is enforced
+  // with no-restricted-imports patterns scoped per glob. Layer order (high->low):
+  // app(exempt top shell) > routes > widgets > features > entities > shared.
+  // A layer bans imports from itself (cross-slice via alias) and every layer above.
+  // Overrides are LAST-WINS, not merged: a file matching two overrides keeps only the
+  // last rule config. So the domain override (below) is a self-contained superset, and
+  // the test/story override (last) fully disables boundary checks for those files.
+  "overrides": [
+    // shared = lowest layer: imports nothing above it
+    {
+      "files": ["src/shared/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": [
+                "$app",
+                "$app/*",
+                "$routes",
+                "$routes/*",
+                "$widgets",
+                "$widgets/*",
+                "$features",
+                "$features/*",
+                "$entities",
+                "$entities/*"
+              ],
+              "message": "FSD layer violation: `shared` is the lowest layer and may not import from any layer above it."
+            }
+          ]
+        }]
+      }
+    },
+    // entities: import shared only; no other entity via alias; interior ui<-only-ui
+    {
+      "files": ["src/entities/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": ["$app", "$app/*", "$routes", "$routes/*", "$widgets", "$widgets/*", "$features", "$features/*"],
+              "message": "FSD layer violation: `entities` may only import from `shared`."
+            },
+            {
+              "group": ["$entities", "$entities/*"],
+              "message": "FSD cross-slice violation: do not import another entity via its alias. Use relative imports inside your own slice; invert the dependency through a higher layer for cross-slice needs."
+            },
+            {
+              "group": ["../ui", "../ui/*", "../../ui/*"],
+              "message": "FSD segment violation: only `ui` may import `ui`. Interior direction is ui -> model -> domain."
+            }
+          ]
+        }]
+      }
+    },
+    // features: import entities/shared only; no other feature via alias
+    {
+      "files": ["src/features/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": ["$app", "$app/*", "$routes", "$routes/*", "$widgets", "$widgets/*"],
+              "message": "FSD layer violation: `features` may only import from `entities` and `shared`."
+            },
+            {
+              "group": ["$features", "$features/*"],
+              "message": "FSD cross-slice violation: do not import another feature via its alias. Invert the dependency through a higher layer (widget/route)."
+            },
+            {
+              "group": ["../ui", "../ui/*", "../../ui/*"],
+              "message": "FSD segment violation: only `ui` may import `ui`. Interior direction is ui -> model -> domain."
+            }
+          ]
+        }]
+      }
+    },
+    // widgets: import features/entities/shared only; no other widget via alias
+    {
+      "files": ["src/widgets/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": ["$app", "$app/*", "$routes", "$routes/*"],
+              "message": "FSD layer violation: `widgets` may only import from `features`, `entities`, and `shared`."
+            },
+            {
+              "group": ["$widgets", "$widgets/*"],
+              "message": "FSD cross-slice violation: do not import another widget via its alias. Invert the dependency through the route layer."
+            },
+            {
+              "group": ["../ui", "../ui/*", "../../ui/*"],
+              "message": "FSD segment violation: only `ui` may import `ui`. Interior direction is ui -> model -> domain."
+            }
+          ]
+        }]
+      }
+    },
+    // routes: top of the FSD list, imports any layer below; only app is above it
+    {
+      "files": ["src/routes/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            { "group": ["$app", "$app/*"], "message": "FSD layer violation: `routes` may not import from `app`." }
+          ]
+        }]
+      }
+    },
+    // domain (FSD+): pure logic. Imports NO layer (not even shared) and no sibling
+    // model/ui segment. Superset: wins over the layer override above for these files.
+    {
+      "files": ["src/**/domain/**"],
+      "rules": {
+        "no-restricted-imports": ["error", {
+          "patterns": [
+            {
+              "group": [
+                "$app",
+                "$app/*",
+                "$routes",
+                "$routes/*",
+                "$widgets",
+                "$widgets/*",
+                "$features",
+                "$features/*",
+                "$entities",
+                "$entities/*",
+                "$shared",
+                "$shared/*"
+              ],
+              "message": "FSD+ domain isolation: `domain` is pure business logic and may not import any layer (including `shared`). Allowed: relative imports within `domain` and framework-agnostic npm packages."
+            },
+            {
+              "group": ["../model", "../model/*", "../../model/*", "../ui", "../ui/*", "../../ui/*"],
+              "message": "FSD+ domain isolation: `domain` may not import sibling `model` or `ui` segments. Dependency flows ui -> model -> domain, never back."
+            }
+          ]
+        }]
+      }
+    },
+    // tests/stories/fixtures legitimately cross-import (e.g. $entities/Font/testing).
+    // Must be LAST so last-wins disables boundary checks for them.
+    {
+      "files": ["**/*.test.ts", "**/*.spec.ts", "**/*.stories.svelte", "src/**/testing/**"],
+      "rules": {
+        "no-restricted-imports": "off"
+      }
+    }
+  ]
+}

.storybook/Decorator.svelte

@@ -0,0 +1,26 @@
+<!--
+    Component: Decorator
+    Global Storybook decorator that wraps all stories with necessary providers.
+
+    This provides:
+    - ResponsiveManager context for breakpoint tracking
+    - TooltipProvider for tooltip components
+-->
+<script lang="ts">
+import { createResponsiveManager } from '$shared/lib';
+import type { ResponsiveManager } from '$shared/lib';
+import { setContext } from 'svelte';
+
+interface Props {
+    children: import('svelte').Snippet;
+}
+
+let { children }: Props = $props();
+
+// Create and provide responsive context
+const responsiveManager = createResponsiveManager();
+$effect(() => responsiveManager.init());
+setContext<ResponsiveManager>('responsive', responsiveManager);
+</script>
+
+{@render children()}

.storybook/StoryStage.svelte

@@ -1,15 +1,19 @@
 <script lang="ts">
 interface Props {
     children: import('svelte').Snippet;
-    width?: string; // Optional width override
+    /**
+     * Tailwind max-width class applied to the card, or 'none' to remove width constraint.
+     * @default 'max-w-3xl'
+     */
+    maxWidth?: string;
 }
 
-let { children, width = 'max-w-3xl' }: Props = $props();
+let { children, maxWidth = 'max-w-3xl' }: Props = $props();
 </script>
 
-<div class="flex min-h-screen w-full items-center justify-center bg-slate-50 p-8">
-    <div class="w-full bg-white shadow-xl ring-1 ring-slate-200 rounded-xl p-12 {width}">
-        <div class="relative flex justify-center items-center">
+<div class="flex min-h-screen w-full items-center justify-center bg-background p-8">
+    <div class="w-full bg-card shadow-lg ring-1 ring-border rounded-xl p-12 {maxWidth !== 'none' ? maxWidth : ''}">
+        <div class="relative flex justify-center items-center text-foreground">
             {@render children()}
         </div>
     </div>

.storybook/ThemeDecorator.svelte

@@ -0,0 +1,79 @@
+<!--
+    Component: ThemeDecorator
+    Storybook decorator that initializes ThemeManager for theme-related stories.
+    Ensures theme management works correctly in Storybook's iframe environment.
+    Includes a floating theme toggle for universal theme switching across all stories.
+-->
+<script lang="ts">
+import { themeManager } from '$features/ChangeAppTheme';
+import type { ResponsiveManager } from '$shared/lib';
+import { IconButton } from '$shared/ui';
+import MoonIcon from '@lucide/svelte/icons/moon';
+import SunIcon from '@lucide/svelte/icons/sun';
+import { getContext } from 'svelte';
+import {
+    onDestroy,
+    onMount,
+} from 'svelte';
+
+interface Props {
+    children: import('svelte').Snippet;
+}
+
+let { children }: Props = $props();
+
+// Get responsive context (set by Decorator)
+const responsive = getContext<ResponsiveManager>('responsive');
+
+// Initialize themeManager on mount
+onMount(() => {
+    themeManager.init();
+
+    // Add keyboard shortcut for theme toggle (Cmd/Ctrl+Shift+D)
+    const handleKeyDown = (e: KeyboardEvent) => {
+        if ((e.metaKey || e.ctrlKey) && e.shiftKey && e.key === 'D') {
+            e.preventDefault();
+            themeManager.toggle();
+        }
+    };
+
+    document.addEventListener('keydown', handleKeyDown);
+
+    return () => {
+        document.removeEventListener('keydown', handleKeyDown);
+    };
+});
+
+// Clean up themeManager when story unmounts
+onDestroy(() => {
+    themeManager.destroy();
+});
+
+const theme = $derived(themeManager.value);
+const themeLabel = $derived(theme === 'light' ? 'Light' : 'Dark');
+</script>
+
+<!-- Floating Theme Toggle -->
+<div
+    class="fixed top-4 right-4 z-50 flex items-center gap-2 px-3 py-2 bg-card border border-border shadow-lg rounded-lg"
+    title="Toggle theme (Cmd/Ctrl+Shift+D)"
+>
+    <span class="text-xs font-medium text-muted-foreground">Theme: {themeLabel}</span>
+    <IconButton
+        onclick={() => themeManager.toggle()}
+        size={responsive?.isMobile ? 'sm' : 'md'}
+        variant="ghost"
+        title="Toggle theme"
+    >
+        {#snippet icon()}
+            {#if theme === 'light'}
+                <MoonIcon class="size-4" />
+            {:else}
+                <SunIcon class="size-4" />
+            {/if}
+        {/snippet}
+    </IconButton>
+</div>
+
+<!-- Story Content -->
+{@render children()}

.storybook/main.ts

@@ -21,7 +21,8 @@ const config: StorybookConfig = {
         {
             name: '@storybook/addon-svelte-csf',
             options: {
-                legacyTemplate: true, // Enables the legacy template syntax
+                // Use modern template syntax for better performance
+                legacyTemplate: false,
             },
         },
         '@chromatic-com/storybook',

.storybook/preview-head.html

@@ -0,0 +1,13 @@
+<link rel="preconnect" href="https://api.fontshare.com" />
+<link rel="preconnect" href="https://cdn.fontshare.com" crossorigin="anonymous" />
+<link rel="preconnect" href="https://fonts.googleapis.com" />
+<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin="anonymous" />
+<link
+    rel="stylesheet"
+    href="https://fonts.googleapis.com/css2?family=Barlow:ital,wght@0,100;0,200;1,100;1,200&family=Karla:wght@200..800&family=Major+Mono+Display&display=swap"
+/>
+<style>
+    body {
+        font-family: "Karla", system-ui, -apple-system, "Segoe UI", Inter, Roboto, Arial, sans-serif;
+    }
+</style>

.storybook/preview.ts

@@ -1,10 +1,12 @@
 import type { Preview } from '@storybook/svelte-vite';
+import Decorator from './Decorator.svelte';
 import StoryStage from './StoryStage.svelte';
+import ThemeDecorator from './ThemeDecorator.svelte';
 import '../src/app/styles/app.css';
 
 const preview: Preview = {
     parameters: {
-        layout: 'fullscreen',
+        layout: 'padded',
         controls: {
             matchers: {
                 color: /(background|color)$/i,
@@ -22,26 +24,122 @@ const preview: Preview = {
         docs: {
             story: {
                 // This sets the default height for the iframe in Autodocs
-                iframeHeight: '400px',
-                // Ensure the story isn't forced into a tiny inline box
-                // inline: true,
+                iframeHeight: '600px',
+            },
+        },
+
+        viewport: {
+            viewports: {
+                // Mobile devices
+                mobile1: {
+                    name: 'iPhone 5/SE',
+                    styles: {
+                        width: '320px',
+                        height: '568px',
+                    },
+                },
+                mobile2: {
+                    name: 'iPhone 14 Pro Max',
+                    styles: {
+                        width: '414px',
+                        height: '896px',
+                    },
+                },
+                // Tablet
+                tablet: {
+                    name: 'iPad (Portrait)',
+                    styles: {
+                        width: '834px',
+                        height: '1112px',
+                    },
+                },
+                desktop: {
+                    name: 'Desktop (Small)',
+                    styles: {
+                        width: '1024px',
+                        height: '1280px',
+                    },
+                },
+                // Widget-specific viewports
+                widgetMedium: {
+                    name: 'Widget Medium',
+                    styles: {
+                        width: '768px',
+                        height: '800px',
+                    },
+                },
+                widgetWide: {
+                    name: 'Widget Wide',
+                    styles: {
+                        width: '1024px',
+                        height: '800px',
+                    },
+                },
+                widgetExtraWide: {
+                    name: 'Widget Extra Wide',
+                    styles: {
+                        width: '1280px',
+                        height: '800px',
+                    },
+                },
+                // Full-width viewports
+                fullWidth: {
+                    name: 'Full Width',
+                    styles: {
+                        width: '100%',
+                        height: '800px',
+                    },
+                },
+                fullScreen: {
+                    name: 'Full Screen',
+                    styles: {
+                        width: '100%',
+                        height: '100%',
                     },
                 },
             },
+        },
+
+        head: `
+            <link rel="preconnect" href="https://api.fontshare.com" />
+            <link rel="preconnect" href="https://cdn.fontshare.com" crossorigin="anonymous" />
+            <link rel="preconnect" href="https://fonts.googleapis.com" />
+            <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin="anonymous" />
+            <link
+                rel="stylesheet"
+                href="https://fonts.googleapis.com/css2?family=Barlow:ital,wght@0,100;0,200;1,100;1,200&family=Karla:wght@200..800&family=Major+Mono+Display&display=swap"
+            />
+            <style>
+                body {
+                    font-family: "Karla", system-ui, -apple-system, "Segoe UI", Inter, Roboto, Arial, sans-serif;
+                }
+            </style>
+        `,
+    },
+
     decorators: [
-        (storyFn, { parameters }) => {
-            const { Component, props } = storyFn();
-            return {
-                Component: StoryStage,
-                // We pass the actual story component into the Stage via a snippet/slot
-                // Svelte 5 Storybook handles this mapping internally when you return this structure
+        // Outermost: initialize ThemeManager for all stories
+        story => ({
+            Component: ThemeDecorator,
             props: {
-                    children: Component,
-                    width: parameters.stageWidth || 'max-w-3xl',
-                    ...props,
+                children: story(),
             },
-            };
+        }),
+        // Wrap with providers (TooltipProvider, ResponsiveManager)
+        story => ({
+            Component: Decorator,
+            props: {
+                children: story(),
             },
+        }),
+        // Wrap with StoryStage for presentation styling
+        (story, context) => ({
+            Component: StoryStage,
+            props: {
+                children: story(),
+                maxWidth: context.parameters.storyStage?.maxWidth,
+            },
+        }),
     ],
 };
 

Caddyfile

@@ -0,0 +1,28 @@
+:3000 {
+    root * /usr/share/caddy
+
+    # Compress text responses only. woff2/png and other binaries are already
+    # compressed, so they're excluded — re-compressing them burns CPU for ~0%.
+    encode {
+        zstd
+        gzip
+        match {
+            header Content-Type text/*
+            header Content-Type application/javascript*
+            header Content-Type application/json*
+            header Content-Type image/svg+xml*
+        }
+    }
+
+    # Vite emits all build output under /assets/ with content-hashed filenames,
+    # so those bytes never change for a given URL — cache them indefinitely.
+    @assets path /assets/*
+    header @assets Cache-Control "public, max-age=31536000, immutable"
+
+    # The HTML shell is the un-hashed entry point; it must revalidate so a new
+    # deploy is served immediately rather than from a stale cache.
+    header /index.html Cache-Control "no-cache"
+
+    try_files {path} /index.html
+    file_server
+}

Dockerfile

@@ -0,0 +1,22 @@
+# Build stage
+FROM node:20-alpine AS builder
+WORKDIR /app
+# Enable Corepack so we can use Yarn v4 (pinned to match lockfile)
+RUN corepack enable && corepack prepare yarn@4.11.0 --activate
+# Force Yarn to use node_modules instead of PnP
+ENV YARN_NODE_LINKER=node-modules
+COPY package.json yarn.lock ./
+RUN yarn install --immutable
+COPY . .
+RUN yarn build && ls -la dist
+
+# Production stage - Caddy
+FROM caddy:2-alpine
+WORKDIR /usr/share/caddy
+# Copy built static files from the builder stage
+COPY --from=builder /app/dist .
+# Copy our local Caddyfile config
+COPY Caddyfile /etc/caddy/Caddyfile
+EXPOSE 3000
+# Start caddy using the config file
+CMD ["caddy", "run", "--config", "/etc/caddy/Caddyfile", "--adapter", "caddyfile"]

README.md

@@ -1,37 +1,38 @@
 # GlyphDiff
 
-A modern, high-performance font exploration tool for browsing and comparing fonts from Google Fonts and Fontshare.
+A modern font exploration and comparison tool for browsing fonts from Google Fonts and Fontshare with real-time visual comparisons, advanced filtering, and customizable typography.
 
-## ✨ Features
+## Features
 
-- **Multi-Provider Support**: Access fonts from Google Fonts and Fontshare in one place
-- **Fast Virtual Scrolling**: Handles thousands of fonts smoothly with custom virtualization
-- **Advanced Filtering**: Filter by category, provider, and character subsets
-- **Responsive Design**: Beautiful UI built with shadcn components and Tailwind CSS
+- **Multi-Provider Catalog**: Browse fonts from Google Fonts and Fontshare in one place
+- **Side-by-Side Comparison**: Compare up to 4 fonts simultaneously with customizable text, size, and typography settings
+- **Advanced Filtering**: Filter by category, provider, character subsets, and weight
+- **Virtual Scrolling**: Fast, smooth browsing of thousands of fonts
+- **Responsive UI**: Beautiful interface built with Tailwind CSS
 - **Type-Safe**: Full TypeScript coverage with strict mode enabled
 
-## 🛠️ Tech Stack
+## Tech Stack
 
-- **Frontend**: Svelte 5 with runes (reactive primitives)
-- **Styling**: Tailwind CSS v4 + shadcn-svelte components
-- **Data Fetching**: TanStack Query for caching and state management
-- **Architecture**: Feature-Sliced Design (FSD) methodology
-- **Testing**: Playwright (E2E), Vitest (unit), Storybook (components)
-- **Quality**: Oxlint (linting), Dprint (formatting), Lefthook (git hooks)
+- **Framework**: Svelte 5 with reactive primitives (runes)
+- **Styling**: Tailwind CSS v4
+- **Components**: Bits UI primitives
+- **State Management**: TanStack Query for async data
+- **Architecture**: Feature-Sliced Design (FSD)
+- **Quality**: oxlint (linting), dprint (formatting), lefthook (git hooks)
 
-## 📁 Architecture
+## Project Structure
 
 ```
 src/
 ├── app/          # App shell, layout, providers
-├── widgets/       # Composed UI blocks
-├── features/      # Business features (filters, search)
-├── entities/      # Domain entities (Font models, stores)
+├── widgets/      # Composed UI blocks (ComparisonSlider, SampleList, FontSearch)
+├── features/     # Business features (filters, search, display)
+├── entities/     # Domain models and stores (Font, Breadcrumb)
 ├── shared/       # Reusable utilities, UI components, helpers
 └── routes/       # Page-level components
 ```
 
-## 🚀 Quick Start
+## Quick Start
 
 ```bash
 # Install dependencies
@@ -40,81 +41,38 @@ yarn install
 # Start development server
 yarn dev
 
-# Open in browser
-yarn dev -- --open
+# Build for production
+yarn build
+
+# Preview production build
+yarn preview
 ```
 
-## 📦 Available Scripts
+## Available Scripts
 
 | Command             | Description                |
-| ---------------- | -------------------------- |
+| ------------------- | -------------------------- |
 | `yarn dev`          | Start development server   |
 | `yarn build`        | Build for production       |
 | `yarn preview`      | Preview production build   |
 | `yarn check`        | Run Svelte type checking   |
 | `yarn lint`         | Run oxlint                 |
-| `yarn format`    | Format with dprint         |
-| `yarn test`      | Run all tests (E2E + unit) |
-| `yarn test:e2e`  | Run Playwright E2E tests   |
-| `yarn test:unit` | Run Vitest unit tests      |
+| `yarn format`       | Format code with dprint    |
+| `yarn test:unit`    | Run unit tests             |
+| `yarn test:unit:ui` | Run Vitest UI              |
 | `yarn storybook`    | Start Storybook dev server |
 
-## 🧪 Development
-
-### Type Checking
-
-```bash
-yarn check           # Single run
-yarn check:watch     # Watch mode
-```
-
-### Testing
-
-```bash
-yarn test:unit              # Unit tests
-yarn test:unit:watch        # Watch mode
-yarn test:unit:ui           # Vitest UI
-yarn test:e2e               # E2E tests with Playwright
-yarn test:e2e --ui          # Interactive test runner
-```
-
-### Code Quality
-
-```bash
-yarn lint          # Lint code
-yarn format        # Format code
-yarn format:check  # Check formatting
-```
-
-## 🎯 Key Components
-
-- **VirtualList**: Custom high-performance list virtualization using Svelte 5 runes
-- **FontList**: Displays fonts with loading, empty, and error states
-- **FilterControls**: Multi-filter system for category, provider, and subsets
-- **TypographyControl**: Dynamic typography adjustment controls
-
-## 📝 Code Style
+## Code Style
 
 - **Path Aliases**: Use `$app/`, `$shared/`, `$features/`, `$entities/`, `$widgets/`, `$routes/`
-- **Components**: PascalCase (e.g., `CheckboxFilter.svelte`)
+- **Components**: PascalCase (e.g., `ComparisonSlider.svelte`)
 - **Formatting**: 100 char line width, 4-space indent, single quotes
-- **Imports**: Auto-sorted by dprint, separated by blank line
-- **Type Safety**: Strict TypeScript, JSDoc comments for public APIs
+- **Type Safety**: Strict TypeScript with JSDoc comments for public APIs
 
-## 🏗️ Building for Production
+## Architecture Notes
 
-```bash
-yarn build
-yarn preview
-```
+This project follows the Feature-Sliced Design (FSD) methodology for clean separation of concerns. The application uses Svelte 5's new runes system (`$state`, `$derived`, `$effect`) for reactive state management.
 
-## 📚 Learn More
-
-- [Svelte 5 Documentation](https://svelte-5-preview.vercel.app/docs)
-- [Feature-Sliced Design](https://feature-sliced.design)
-- [Tailwind CSS v4](https://tailwindcss.com/blog/tailwindcss-v4-alpha)
-- [shadcn-svelte](https://www.shadcn-svelte.com)
-
-## 📄 License
+## License
 
 MIT

components.json

@@ -1,16 +0,0 @@
-{
-  "$schema": "https://shadcn-svelte.com/schema.json",
-  "tailwind": {
-    "css": "src/app.css",
-    "baseColor": "zinc"
-  },
-  "aliases": {
-    "components": "$shared/shadcn/ui",
-    "utils": "$shared/shadcn/utils/shadcn-utils",
-    "ui": "$shared/shadcn/ui",
-    "hooks": "$shared/shadcn/hooks",
-    "lib": "$shared"
-  },
-  "typescript": true,
-  "registry": "https://shadcn-svelte.com/registry"
-}

docs/GIT_WORKFLOW.md

@@ -1,592 +0,0 @@
-# Git Workflow and Branching Strategy
-
-This document outlines the git workflow, branching strategy, commit conventions, and code review guidelines for the glyphdiff.com project.
-
-## Table of Contents
-
-1. [Branching Strategy](#branching-strategy)
-2. [Branch Naming Conventions](#branch-naming-conventions)
-3. [Commit Message Conventions](#commit-message-conventions)
-4. [Code Splitting and Merge Request Guidelines](#code-splitting-and-merge-request-guidelines)
-5. [Branch Protection Rules](#branch-protection-rules)
-6. [Git Hooks Configuration](#git-hooks-configuration)
-
----
-
-## Branching Strategy
-
-We use a Gitflow-inspired branching strategy adapted for our development workflow. This strategy provides a clear structure for feature development, bug fixes, and releases.
-
-### Branch Types
-
-#### 1. `main` Branch
-- **Purpose**: Production-ready code only
-- **Protection**: Highest level of protection
-- **Rules**:
-  - Only merge `release/*` or `hotfix/*` branches into `main`
-  - No direct commits allowed
-  - Must pass all tests and code reviews
-  - Tags are created from this branch for releases (e.g., `v1.0.0`)
-
-#### 2. `develop` Branch
-- **Purpose**: Integration branch for features
-- **Protection**: High level of protection
-- **Rules**:
-  - Merge `feature/*` and `fix/*` branches into `develop`
-  - No direct commits allowed
-  - Must pass all tests before merging
-  - Serves as the base for `release/*` branches
-
-#### 3. `feature/*` Branches
-- **Purpose**: Develop new features
-- **Naming**: `feature/feature-name` (e.g., `feature/font-catalog`, `feature/comparison-grid`)
-- **Base**: Always branch from `develop`
-- **Merge**: Merge back into `develop` via Merge Request (MR)
-- **Rules**:
-  - One feature per branch
-  - Keep branches focused and small
-  - Delete after merging
-
-#### 4. `fix/*` Branches
-- **Purpose**: Fix bugs discovered during development
-- **Naming**: `fix/issue-description` (e.g., `fix/font-loading-error`, `fix/responsive-layout`)
-- **Base**: Branch from `develop`
-- **Merge**: Merge back into `develop` via MR
-- **Rules**:
-  - One fix per branch
-  - Include tests that verify the fix
-  - Delete after merging
-
-#### 5. `hotfix/*` Branches
-- **Purpose**: Critical fixes for production issues
-- **Naming**: `hotfix/critical-fix` (e.g., `hotfix/security-patch`, `hotfix-production-crash`)
-- **Base**: Branch from `main`
-- **Merge**: Merge into both `main` and `develop`
-- **Rules**:
-  - Use only for production emergencies
-  - Must be thoroughly tested
-  - Create a release tag after merging to `main`
-
-#### 6. `release/*` Branches
-- **Purpose**: Prepare for a new release
-- **Naming**: `release/vX.Y.Z` (e.g., `release/v1.0.0`, `release/v1.1.0`)
-- **Base**: Branch from `develop`
-- **Merge**: Merge into both `main` and `develop`
-- **Rules**:
-  - Finalize release notes
-  - Update version numbers
-  - Perform final testing
-  - Create release tag after merging to `main`
-
-### Branch Workflow Diagram
-
-```
-main (production)
-  ↑
-  │ hotfix/*, release/*
-  │
-develop (integration)
-  ↑
-  │ feature/*, fix/*
-  │
-feature branches
-```
-
----
-
-## Branch Naming Conventions
-
-### Feature Branches
-- Format: `feature/feature-name`
-- Examples:
-  - `feature/font-catalog`
-  - `feature/comparison-grid`
-  - `feature/dark-mode`
-  - `feature/google-fonts-integration`
-
-### Fix Branches
-- Format: `fix/issue-description`
-- Examples:
-  - `fix/font-loading-error`
-  - `fix/responsive-layout`
-  - `fix/state-persistence`
-  - `fix-accessibility-contrast`
-
-### Hotfix Branches
-- Format: `hotfix/critical-fix`
-- Examples:
-  - `hotfix/security-patch`
-  - `hotfix-production-crash`
-  - `hotfix-api-rate-limit`
-
-### Release Branches
-- Format: `release/vX.Y.Z`
-- Examples:
-  - `release/v1.0.0`
-  - `release/v1.1.0`
-  - `release/v2.0.0`
-
-### Naming Guidelines
-- Use lowercase letters
-- Use hyphens to separate words
-- Be descriptive but concise
-- Avoid special characters (except hyphens)
-- Keep names under 50 characters
-
----
-
-## Commit Message Conventions
-
-We follow the [Conventional Commits](https://www.conventionalcommits.org/) specification. This format enables automated changelog generation and better commit history readability.
-
-### Format
-
-```
-<type>(<scope>): <subject>
-
-<body>
-
-<footer>
-```
-
-### Commit Types
-
-| Type | Description | Examples |
-|------|-------------|----------|
-| `feat` | New feature | `feat(fonts): add Google Fonts integration` |
-| `fix` | Bug fix | `fix(comparison): resolve font loading race condition` |
-| `docs` | Documentation changes | `docs(readme): update installation instructions` |
-| `style` | Code style changes (formatting, etc.) | `style(components): format with Prettier` |
-| `refactor` | Code refactoring | `refactor(stores): simplify state management` |
-| `test` | Adding or updating tests | `test(fonts): add unit tests for font mapper` |
-| `chore` | Maintenance tasks | `chore(deps): update Tailwind CSS to v4.0` |
-| `perf` | Performance improvements | `perf(catalog): implement lazy loading for fonts` |
-
-### Scope
-
-The scope provides context about which part of the codebase is affected. Common scopes for this project:
-
-- `fonts` - Font-related functionality
-- `comparison` - Font comparison features
-- `catalog` - Font catalog pages
-- `stores` - State management stores
-- `components` - UI components
-- `routes` - SvelteKit routes
-- `services` - External API services
-- `utils` - Utility functions
-- `types` - TypeScript type definitions
-- `ui` - UI-related changes (theme, layout, etc.)
-- `config` - Configuration files
-
-### Subject
-
-- Use imperative mood ("add" not "added", "fix" not "fixed")
-- Keep it short (50 characters or less)
-- Don't end with a period
-- Be specific and descriptive
-
-### Body
-
-- Use imperative mood
-- Explain **what** and **why**, not **how**
-- Wrap at 72 characters
-- Include references to issues (e.g., `Closes #123`)
-
-### Footer
-
-- Reference breaking changes with `BREAKING CHANGE:`
-- Reference issues with `Closes #123` or `Fixes #456`
-- Include co-authors if needed
-
-### Examples
-
-#### Feature Commit
-```
-feat(fonts): add Google Fonts API integration
-
-Implement Google Fonts API service to fetch and display available fonts.
-This includes the fetchGoogleFonts function and font mapper utilities.
-
-Closes #12
-```
-
-#### Bug Fix Commit
-```
-fix(comparison): resolve font loading race condition
-
-The comparison grid was attempting to render fonts before they were fully
-loaded. Added loading state checks to prevent this issue.
-
-Fixes #45
-```
-
-#### Refactor Commit
-```
-refactor(stores): simplify state management with Svelte 5 runes
-
-Migrated from Svelte stores to Svelte 5's $state runes for better
-performance and simpler code. This change affects all stores in the
-project.
-
-BREAKING CHANGE: Store API has changed from subscribe() to direct
-property access. Update all store consumers accordingly.
-```
-
-#### Documentation Commit
-```
-docs(git-workflow): add commit message conventions
-
-Document the conventional commits format with examples and guidelines
-for the team.
-```
-
-#### Chore Commit
-```
-chore(deps): update Tailwind CSS to v4.0.0
-
-Update Tailwind CSS to the latest version and adjust configuration
-files accordingly.
-```
-
----
-
-## Code Splitting and Merge Request Guidelines
-
-### Merge Request Size Guidelines
-
-- **Maximum MR size**: < 500 lines changed (additions + deletions)
-- **Ideal MR size**: 100-300 lines changed
-- **Files per MR**: < 10 files
-
-### When to Split a Feature into Multiple MRs
-
-Split a feature into multiple MRs when:
-
-1. **The feature is large** (> 500 lines or > 10 files)
-2. **Multiple concerns are involved** (e.g., UI + API + state management)
-3. **Independent parts can be tested separately**
-4. **The feature has logical phases** (e.g., setup → implementation → polish)
-
-### Example: Splitting a Feature
-
-**Feature**: Font Catalog with Filtering
-
-**MR 1**: `feature/font-catalog-setup`
-- Create basic catalog page structure
-- Set up routing
-- Add placeholder components
-- ~150 lines
-
-**MR 2**: `feature/font-catalog-data`
-- Implement Google Fonts API integration
-- Create font data fetching logic
-- Add font mapper utilities
-- ~200 lines
-
-**MR 3**: `feature/font-catalog-ui`
-- Build FontCard component
-- Implement grid layout
-- Add loading states
-- ~250 lines
-
-**MR 4**: `feature/font-catalog-filtering`
-- Implement filter store
-- Build FilterBar component
-- Connect filters to catalog
-- ~180 lines
-
-### Merge Request Description Template
-
-Every MR must include a comprehensive description:
-
-```markdown
-## Description
-Brief description of what this MR changes and why.
-
-## Changes Made
-- [ ] Change 1
-- [ ] Change 2
-- [ ] Change 3
-
-## Type of Change
-- [ ] Bug fix
-- [ ] New feature
-- [ ] Breaking change
-- [ ] Documentation update
-- [ ] Refactoring
-- [ ] Performance improvement
-
-## Testing
-- [ ] Unit tests pass
-- [ ] Manual testing completed
-- [ ] Tested on Chrome
-- [ ] Tested on Firefox
-- [ ] Tested on Safari
-- [ ] Tested on mobile (responsive)
-
-## Screenshots (if applicable)
-Add screenshots or GIFs showing the changes.
-
-## Checklist
-- [ ] Code follows project style guidelines
-- [ ] Self-review completed
-- [ ] Comments added for complex logic
-- [ ] Documentation updated
-- [ ] No new warnings generated
-- [ ] Tests added/updated
-- [ ] All tests passing
-
-## Related Issues
-Closes #123
-Related to #456
-```
-
-### Code Review Checklist
-
-Reviewers should check:
-
-#### Functionality
-- [ ] Does the code work as intended?
-- [ ] Are edge cases handled?
-- [ ] Is error handling appropriate?
-
-#### Code Quality
-- [ ] Is the code readable and maintainable?
-- [ ] Are variable/function names descriptive?
-- [ ] Is there unnecessary complexity?
-- [ ] Are there code duplications?
-
-#### Best Practices
-- [ ] Does it follow project conventions?
-- [ ] Are TypeScript types properly defined?
-- [ ] Are Svelte best practices followed?
-- [ ] Is Tailwind CSS used appropriately?
-
-#### Testing
-- [ ] Are tests included?
-- [ ] Do tests cover edge cases?
-- [ ] Are tests meaningful and not redundant?
-
-#### Documentation
-- [ ] Is the code self-documenting?
-- [ ] Are complex functions commented?
-- [ ] Is the MR description clear?
-
-#### Performance
-- [ ] Are there performance concerns?
-- [ ] Is lazy loading used where appropriate?
-- [ ] Are unnecessary re-renders avoided?
-
-### Merge Request Approval Process
-
-1. **Author**: Creates MR with complete description
-2. **Reviewer**: Reviews code using the checklist above
-3. **Discussion**: Address any concerns or suggestions
-4. **Approval**: At least one approval required
-4. **Merge**: Squash and merge into target branch
-5. **Cleanup**: Delete source branch after merge
-
----
-
-## Branch Protection Rules
-
-### `main` Branch Protection
-
-- **Require pull request reviews**: Yes
-  - Required approvers: 1
-  - Dismiss stale reviews: Yes
-- **Require status checks**: Yes
-  - Required checks: All tests, linting
-  - Require branches to be up to date: Yes
-- **Restrict who can push**: Only maintainers
-- **Require linear history**: Yes (squash and merge)
-- **Block force pushes**: Yes
-
-### `develop` Branch Protection
-
-- **Require pull request reviews**: Yes
-  - Required approvers: 1
-  - Dismiss stale reviews: Yes
-- **Require status checks**: Yes
-  - Required checks: All tests, linting
-  - Require branches to be up to date: Yes
-- **Restrict who can push**: Only developers and maintainers
-- **Require linear history**: Yes (squash and merge)
-- **Block force pushes**: Yes
-
-### Implementation Notes
-
-These rules should be configured in your Git hosting platform (GitHub, GitLab, or Bitbucket). The exact configuration steps vary by platform:
-
-- **GitHub**: Settings → Branches → Add rule
-- **GitLab**: Settings → Repository → Protected branches
-- **Bitbucket**: Repository settings → Branch restrictions
-
----
-
-## Git Hooks Configuration
-
-Git hooks are automated scripts that run at specific points in the git workflow. They help maintain code quality and consistency.
-
-### Recommended Hooks
-
-#### 1. Pre-commit Hook
-**Purpose**: Run linter and formatter before committing
-
-**Tools**: ESLint, Prettier
-
-**Implementation**:
-```bash
-#!/bin/sh
-# .git/hooks/pre-commit
-
-# Run Prettier
-npm run format:check
-
-# Run ESLint
-npm run lint
-
-# Exit with error if any check fails
-if [ $? -ne 0 ]; then
-  echo "❌ Pre-commit checks failed. Please fix the issues before committing."
-  exit 1
-fi
-
-echo "✅ Pre-commit checks passed."
-```
-
-**Setup**:
-```bash
-# Install husky (recommended)
-npm install --save-dev husky
-
-# Initialize husky
-npx husky install
-
-# Add pre-commit hook
-npx husky add .husky/pre-commit "npm run lint && npm run format:check"
-```
-
-#### 2. Commit-msg Hook
-**Purpose**: Validate commit message format
-
-**Tools**: commitlint
-
-**Implementation**:
-```bash
-#!/bin/sh
-# .git/hooks/commit-msg
-
-# Validate commit message with commitlint
-npx --no -- commitlint --edit $1
-```
-
-**Setup**:
-```bash
-# Install commitlint
-npm install --save-dev @commitlint/cli @commitlint/config-conventional
-
-# Create commitlint config
-echo "module.exports = { extends: ['@commitlint/config-conventional'] };" > commitlint.config.js
-
-# Add commit-msg hook
-npx husky add .husky/commit-msg "npx --no -- commitlint --edit \$1"
-```
-
-#### 3. Pre-push Hook
-**Purpose**: Run tests before pushing
-
-**Tools**: Vitest, SvelteKit test runner
-
-**Implementation**:
-```bash
-#!/bin/sh
-# .git/hooks/pre-push
-
-# Run tests
-npm run test
-
-# Exit with error if tests fail
-if [ $? -ne 0 ]; then
-  echo "❌ Tests failed. Please fix the failing tests before pushing."
-  exit 1
-fi
-
-echo "✅ All tests passed."
-```
-
-**Setup**:
-```bash
-# Add pre-push hook
-npx husky add .husky/pre-push "npm run test"
-```
-
-### Alternative: Using Husky
-
-[Husky](https://typicode.github.io/husky/) is a popular tool for managing git hooks. It's easier to maintain and works across different operating systems.
-
-**Installation**:
-```bash
-npm install --save-dev husky
-npx husky install
-npm pkg set scripts.prepare="husky install"
-```
-
-**Adding hooks**:
-```bash
-# Pre-commit hook
-npx husky add .husky/pre-commit "npm run lint && npm run format:check"
-
-# Commit-msg hook
-npx husky add .husky/commit-msg "npx --no -- commitlint --edit \$1"
-
-# Pre-push hook
-npx husky add .husky/pre-push "npm run test"
-```
-
-### Hook Scripts for This Project
-
-Once the project is set up with SvelteKit, add these scripts to `package.json`:
-
-```json
-{
-  "scripts": {
-    "format": "prettier --write .",
-    "format:check": "prettier --check .",
-    "lint": "eslint .",
-    "lint:fix": "eslint . --fix",
-    "test": "vitest run",
-    "test:watch": "vitest",
-    "prepare": "husky install"
-  }
-}
-```
-
-### Benefits of Git Hooks
-
-1. **Consistency**: Enforce code style and formatting
-2. **Quality**: Catch bugs before they're committed
-3. **Efficiency**: Fail fast, fix early
-4. **Automation**: Reduce manual checks
-5. **Team alignment**: Ensure everyone follows the same standards
-
----
-
-## Summary
-
-This git workflow provides a structured approach to development for the glyphdiff.com project:
-
-- **Clear branching strategy** with defined purposes for each branch type
-- **Conventional commits** for readable and automated changelogs
-- **Code splitting guidelines** to keep MRs focused and reviewable
-- **Comprehensive review process** to maintain code quality
-- **Git hooks** to automate quality checks
-
-Following this workflow will help the team:
-- Develop features in parallel without conflicts
-- Maintain a clean git history
-- Catch issues early in the development process
-- Ensure code quality and consistency
-- Streamline the release process
-
-For questions or suggestions about this workflow, please discuss with the team or create an issue in the project repository.

dprint.json

@@ -13,7 +13,7 @@
     "https://plugins.dprint.dev/typescript-0.93.0.wasm",
     "https://plugins.dprint.dev/json-0.19.3.wasm",
     "https://plugins.dprint.dev/markdown-0.17.8.wasm",
-    "https://plugins.dprint.dev/g-plane/markup_fmt-v0.25.3.wasm"
+    "https://plugins.dprint.dev/g-plane/markup_fmt-v0.27.0.wasm"
   ],
   "typescript": {
     "lineWidth": 120,
@@ -31,7 +31,17 @@
     "importDeclaration.forceMultiLine": "whenMultiple",
     "importDeclaration.forceSingleLine": false,
     "exportDeclaration.forceMultiLine": "whenMultiple",
-    "exportDeclaration.forceSingleLine": false
+    "exportDeclaration.forceSingleLine": false,
+    "ifStatement.useBraces": "always",
+    "ifStatement.singleBodyPosition": "nextLine",
+    "whileStatement.useBraces": "always",
+    "whileStatement.singleBodyPosition": "nextLine",
+    "forStatement.useBraces": "always",
+    "forStatement.singleBodyPosition": "nextLine",
+    "forInStatement.useBraces": "always",
+    "forInStatement.singleBodyPosition": "nextLine",
+    "forOfStatement.useBraces": "always",
+    "forOfStatement.singleBodyPosition": "nextLine"
   },
   "json": {
     "indentWidth": 2,
@@ -47,9 +57,8 @@
     "quotes": "double",
     "scriptIndent": false,
     "styleIndent": false,
-
-    "vBindStyle": "short",
-    "vOnStyle": "short",
-    "formatComments": true
+    "formatComments": true,
+    "svelteAttrShorthand": true,
+    "svelteDirectiveShorthand": true
   }
 }

e2e/compare-flow.test.ts

@@ -0,0 +1,39 @@
+import {
+    expect,
+    test,
+} from './fixtures';
+
+test.describe('compare flow', () => {
+    test('selects fontA and fontB onto opposite sides', async ({ comparison }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+
+        // Each side's header region exposes the font name independently.
+        await expect(comparison.primaryFont).toContainText('Inter');
+        await expect(comparison.secondaryFont).toContainText('Roboto');
+
+        // Slider is rendered and interactive once both fonts are picked.
+        await expect(comparison.slider).toBeVisible();
+    });
+
+    test('reflects active side via aria-pressed', async ({ comparison }) => {
+        await comparison.selectSide('B');
+        expect(await comparison.activeSide()).toBe('B');
+        await expect(comparison.secondarySideButton).toHaveAttribute('aria-pressed', 'true');
+        await expect(comparison.primarySideButton).toHaveAttribute('aria-pressed', 'false');
+    });
+
+    test('persists selection through the comparisonStore localStorage', async ({ comparison }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+
+        // Wait for the store debounce to flush to localStorage.
+        await expect.poll(async () => {
+            const storage = await comparison.readStorage();
+            return storage['glyphdiff:comparison'];
+        }).toMatch(/inter/i);
+
+        const storage = await comparison.readStorage();
+        const state = JSON.parse(storage['glyphdiff:comparison']!);
+        expect(state.fontAId).toBe('inter');
+        expect(state.fontBId).toBe('roboto');
+    });
+});

e2e/fixtures.ts

@@ -0,0 +1,35 @@
+import { test as base } from '@playwright/test';
+import { ComparisonPage } from './pages/comparison-page';
+import { TypographyMenu } from './pages/typography-menu';
+
+type Fixtures = {
+    /**
+     * Opened ComparisonPage with the root view loaded.
+     */
+    comparison: ComparisonPage;
+    /**
+     * Typography menu helper bound to the same page.
+     */
+    typography: TypographyMenu;
+};
+
+/**
+ * Custom test that auto-opens the comparison view before each spec.
+ * Playwright gives each test a fresh BrowserContext by default, so
+ * localStorage is empty unless a test seeds it.
+ */
+export const test = base.extend<Fixtures>({
+    comparison: async ({ page }, use) => {
+        const view = new ComparisonPage(page);
+        await view.open();
+        await use(view);
+    },
+    // Depends on `comparison` so the root page is opened before the menu is
+    // consulted — TypographyMenu has no markup of its own to load.
+    typography: async ({ comparison, page }, use) => {
+        void comparison;
+        await use(new TypographyMenu(page));
+    },
+});
+
+export { expect } from '@playwright/test';

e2e/font-loading.test.ts

@@ -0,0 +1,22 @@
+import {
+    expect,
+    test,
+} from './fixtures';
+
+test.describe('font loading', () => {
+    test('selected fonts land in the FontFaceSet with status="loaded"', async ({ comparison }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+
+        await expect.poll(() => comparison.fontLoaded('Inter')).toBe(true);
+        await expect.poll(() => comparison.fontLoaded('Roboto')).toBe(true);
+    });
+
+    test('an unrelated font remains absent from the FontFaceSet', async ({ comparison }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+
+        // "Audiowide" is unlikely to be on the system AND was not selected, so
+        // no FontFace should ever have been registered for it. This guards
+        // against the loader over-fetching neighbouring fonts.
+        await expect.poll(() => comparison.fontLoaded('Audiowide')).toBe(false);
+    });
+});

e2e/pages/base-page.ts

@@ -0,0 +1,16 @@
+import type { Page } from '@playwright/test';
+
+/**
+ * Shared base for all page objects. Subclasses extend this and expose
+ * domain-specific locators + actions — never raw selectors leaking into tests.
+ */
+export abstract class BasePage {
+    protected constructor(protected readonly page: Page) {}
+
+    /**
+     * Navigate to a path relative to baseURL.
+     */
+    async goto(path = '/') {
+        await this.page.goto(path);
+    }
+}

e2e/pages/comparison-page.ts

@@ -0,0 +1,144 @@
+import type {
+    Locator,
+    Page,
+} from '@playwright/test';
+import { BasePage } from './base-page';
+
+/**
+ * Page object for the root comparison view. Encapsulates locators for the
+ * primary controls so tests don't hardcode aria-labels or DOM structure.
+ *
+ * Selection flow: clicking a font row assigns it to whichever side
+ * (`A` = "Left Font" / Primary, `B` = "Right Font" / Secondary) is currently
+ * active in the Sidebar — there's no per-row A/B toggle.
+ */
+export class ComparisonPage extends BasePage {
+    readonly searchInput: Locator;
+    readonly previewInput: Locator;
+    readonly slider: Locator;
+    readonly primarySideButton: Locator;
+    readonly secondarySideButton: Locator;
+    readonly primaryFont: Locator;
+    readonly secondaryFont: Locator;
+    readonly fontList: Locator;
+
+    constructor(page: Page) {
+        super(page);
+        this.searchInput = page.getByRole('textbox', { name: 'Search typefaces' });
+        this.previewInput = page.getByRole('textbox', { name: 'Preview text' });
+        this.slider = page.getByRole('slider', { name: 'Font comparison slider' });
+        // ARIA-controls couples the side toggle to the font display it targets — copy-independent.
+        this.primarySideButton = page.locator('[aria-controls="primary-font"]');
+        this.secondarySideButton = page.locator('[aria-controls="secondary-font"]');
+        this.primaryFont = page.locator('#primary-font');
+        this.secondaryFont = page.locator('#secondary-font');
+        this.fontList = page.locator('[data-font-list]');
+    }
+
+    /**
+     * Open the root page and wait for the main controls to be interactable.
+     * Uses lg+ viewport for the preview input to be visible.
+     */
+    async open() {
+        await this.goto('/');
+        await this.searchInput.waitFor({ state: 'visible' });
+    }
+
+    async searchFor(query: string) {
+        await this.searchInput.fill(query);
+    }
+
+    async setPreviewText(text: string) {
+        await this.previewInput.fill(text);
+    }
+
+    /**
+     * Switch which side the next font click will assign to.
+     */
+    async selectSide(side: 'A' | 'B') {
+        const button = side === 'A' ? this.primarySideButton : this.secondarySideButton;
+        await button.click();
+    }
+
+    /**
+     * Read which side is currently active from `aria-pressed`.
+     * Falls back to A when neither button reports pressed (initial state in some flows).
+     */
+    async activeSide(): Promise<'A' | 'B' | null> {
+        const [primaryPressed, secondaryPressed] = await Promise.all([
+            this.primarySideButton.getAttribute('aria-pressed'),
+            this.secondarySideButton.getAttribute('aria-pressed'),
+        ]);
+        if (primaryPressed === 'true') {
+            return 'A';
+        }
+        if (secondaryPressed === 'true') {
+            return 'B';
+        }
+        return null;
+    }
+
+    /**
+     * Search for a font and click the matching list row. The row's accessible
+     * name is the font name itself (rendered by FontApplicator).
+     */
+    async pickFont(name: string) {
+        await this.searchFor(name);
+        const row = this.fontList.getByRole('button', { name, exact: true });
+        await row.click();
+    }
+
+    /**
+     * Assign fontA to side A and fontB to side B in one call.
+     */
+    async pickPair(fontA: string, fontB: string) {
+        await this.selectSide('A');
+        await this.pickFont(fontA);
+        await this.selectSide('B');
+        await this.pickFont(fontB);
+    }
+
+    /**
+     * Read aria-valuenow off the comparison slider.
+     */
+    async sliderValue(): Promise<number> {
+        const value = await this.slider.getAttribute('aria-valuenow');
+        return Number(value);
+    }
+
+    /**
+     * Snapshot the glyphdiff:* localStorage entries.
+     */
+    async readStorage(): Promise<Record<string, string | null>> {
+        return await this.page.evaluate(() => {
+            const out: Record<string, string | null> = {};
+            for (let i = 0; i < localStorage.length; i++) {
+                const key = localStorage.key(i)!;
+                if (key.startsWith('glyphdiff:')) {
+                    out[key] = localStorage.getItem(key);
+                }
+            }
+            return out;
+        });
+    }
+
+    /**
+     * Whether the document.fonts FontFaceSet contains a fully-loaded face for
+     * the named family. Counts only faces registered via the FontFace API —
+     * system-installed fallbacks (which `document.fonts.check` honours) are
+     * excluded, so a `false` here is meaningful in negative assertions.
+     */
+    async fontLoaded(name: string): Promise<boolean> {
+        return await this.page.evaluate(target => {
+            for (const face of document.fonts) {
+                // FontFace.family is wrapped in quotes only if the literal was;
+                // strip any surrounding quotes before comparing.
+                const family = face.family.replace(/^["']|["']$/g, '');
+                if (family === target && face.status === 'loaded') {
+                    return true;
+                }
+            }
+            return false;
+        }, name);
+    }
+}

e2e/pages/typography-menu.ts

@@ -0,0 +1,73 @@
+import type {
+    Locator,
+    Page,
+} from '@playwright/test';
+
+/**
+ * Typography settings menu — desktop layout exposes inline ComboControls with
+ * increase/decrease buttons. The current value is encoded in the trigger
+ * button's aria-label as `${controlLabel}: ${value}` (e.g. "Size: 24").
+ */
+export type TypographyControl = 'size' | 'weight' | 'leading' | 'tracking';
+
+const LABELS: Record<TypographyControl, { increase: string; decrease: string; trigger: string }> = {
+    size: {
+        increase: 'Increase Font Size',
+        decrease: 'Decrease Font Size',
+        trigger: 'Size',
+    },
+    weight: {
+        increase: 'Increase Font Weight',
+        decrease: 'Decrease Font Weight',
+        trigger: 'Weight',
+    },
+    leading: {
+        increase: 'Increase Line Height',
+        decrease: 'Decrease Line Height',
+        trigger: 'Leading',
+    },
+    tracking: {
+        increase: 'Increase Letter Spacing',
+        decrease: 'Decrease Letter Spacing',
+        trigger: 'Tracking',
+    },
+};
+
+export class TypographyMenu {
+    constructor(private readonly page: Page) {}
+
+    increase(control: TypographyControl): Locator {
+        return this.page.getByRole('button', { name: LABELS[control].increase });
+    }
+
+    decrease(control: TypographyControl): Locator {
+        return this.page.getByRole('button', { name: LABELS[control].decrease });
+    }
+
+    /**
+     * Trigger button whose aria-label encodes the current value, e.g. "Size: 24".
+     */
+    trigger(control: TypographyControl): Locator {
+        return this.page.getByRole('button', { name: new RegExp(`^${LABELS[control].trigger}:\\s`) });
+    }
+
+    /**
+     * Parse the numeric value out of the trigger button's aria-label.
+     * Returns null if the label can't be read yet.
+     */
+    async readValue(control: TypographyControl): Promise<number | null> {
+        const label = await this.trigger(control).getAttribute('aria-label');
+        if (!label) {
+            return null;
+        }
+        const match = label.match(/:\s*(-?\d+(?:\.\d+)?)/);
+        return match ? Number(match[1]) : null;
+    }
+
+    async bump(control: TypographyControl, direction: 'up' | 'down', times = 1) {
+        const button = direction === 'up' ? this.increase(control) : this.decrease(control);
+        for (let i = 0; i < times; i++) {
+            await button.click();
+        }
+    }
+}

e2e/persistence.test.ts

@@ -0,0 +1,41 @@
+import {
+    expect,
+    test,
+} from './fixtures';
+
+test.describe('persistence', () => {
+    test('restores selected fonts after reload', async ({ comparison, page }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+
+        // Confirm the store has flushed before reloading — otherwise we race
+        // the debounce and may reload with empty storage.
+        await expect.poll(async () => {
+            const storage = await comparison.readStorage();
+            return storage['glyphdiff:comparison'];
+        }).toMatch(/roboto/i);
+
+        await page.reload();
+        await comparison.searchInput.waitFor({ state: 'visible' });
+
+        await expect(comparison.primaryFont).toContainText('Inter');
+        await expect(comparison.secondaryFont).toContainText('Roboto');
+    });
+
+    test('restores typography settings after reload', async ({ comparison, typography, page }) => {
+        const baseline = await typography.readValue('size');
+        await typography.bump('size', 'up', 2);
+
+        const bumped = await typography.readValue('size');
+        expect(bumped).not.toBe(baseline);
+
+        await expect.poll(async () => {
+            const storage = await comparison.readStorage();
+            return storage['glyphdiff:comparison:typography'];
+        }).not.toBeNull();
+
+        await page.reload();
+        await comparison.searchInput.waitFor({ state: 'visible' });
+
+        expect(await typography.readValue('size')).toBe(bumped);
+    });
+});

e2e/preview-text.test.ts

@@ -0,0 +1,32 @@
+import { windowSizeForLine } from '../src/entities/Font/domain/windowSizeForLine/windowSizeForLine';
+import {
+    expect,
+    test,
+} from './fixtures';
+
+test.describe('preview text', () => {
+    test('drives the slider character rendering', async ({ comparison }) => {
+        /**
+         * Must stay a single unwrapped line of ASCII: the assertion feeds
+         * `text.length` (UTF-16 code units) to `windowSizeForLine`, but the
+         * renderer feeds it the line's grapheme count. They match only for
+         * plain ASCII — emoji/combining marks (length > graphemes) or wrapping
+         * (one input string splitting into several lines) silently desync them.
+         */
+        const text = 'Sphinx';
+        await comparison.pickPair('Inter', 'Roboto');
+        await comparison.setPreviewText(text);
+
+        // Window chars render as `.char-wrap` cells for crossfade. The window
+        // size is a pure function of the line's grapheme count — assert against
+        // the rule, not a hardcoded constant, so tuning the policy can't silently
+        // break this. "Sphinx" is one unwrapped line of 6 graphemes.
+        await expect(comparison.slider.locator('.char-wrap')).toHaveCount(windowSizeForLine(text.length));
+    });
+
+    test('preserves the typed value in the input', async ({ comparison }) => {
+        const text = 'Sphinx of black quartz';
+        await comparison.setPreviewText(text);
+        await expect(comparison.previewInput).toHaveValue(text);
+    });
+});

e2e/slider.test.ts

@@ -0,0 +1,46 @@
+import {
+    expect,
+    test,
+} from './fixtures';
+
+/**
+ * Slider position is spring-animated; aria-valuenow reflects the current
+ * value, not the target. All assertions use `toHaveAttribute` so Playwright
+ * polls until the spring settles.
+ */
+test.describe('comparison slider', () => {
+    test.beforeEach(async ({ comparison }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+        await comparison.slider.focus();
+    });
+
+    test('keyboard navigation snaps to End and Home', async ({ comparison }) => {
+        await comparison.slider.press('End');
+        await expect(comparison.slider).toHaveAttribute('aria-valuenow', '100');
+
+        await comparison.slider.press('Home');
+        await expect(comparison.slider).toHaveAttribute('aria-valuenow', '0');
+    });
+
+    test('arrow keys nudge by one, Shift+Arrow by ten', async ({ comparison }) => {
+        await comparison.slider.press('Home');
+        await expect(comparison.slider).toHaveAttribute('aria-valuenow', '0');
+
+        await comparison.slider.press('ArrowRight');
+        await expect(comparison.slider).toHaveAttribute('aria-valuenow', '1');
+
+        await comparison.slider.press('Shift+ArrowRight');
+        await expect(comparison.slider).toHaveAttribute('aria-valuenow', '11');
+    });
+
+    test('PageUp / PageDown move by ten', async ({ comparison }) => {
+        await comparison.slider.press('Home');
+        await expect(comparison.slider).toHaveAttribute('aria-valuenow', '0');
+
+        await comparison.slider.press('PageUp');
+        await expect(comparison.slider).toHaveAttribute('aria-valuenow', '10');
+
+        await comparison.slider.press('PageDown');
+        await expect(comparison.slider).toHaveAttribute('aria-valuenow', '0');
+    });
+});

e2e/smoke.test.ts

@@ -0,0 +1,23 @@
+import {
+    expect,
+    test,
+} from '@playwright/test';
+import { ComparisonPage } from './pages/comparison-page';
+
+test.describe('smoke', () => {
+    test('loads the comparison view with its primary controls', async ({ page }) => {
+        const view = new ComparisonPage(page);
+        await view.open();
+
+        await expect(view.searchInput).toBeVisible();
+        await expect(view.previewInput).toBeVisible();
+    });
+
+    test('accepts a search query', async ({ page }) => {
+        const view = new ComparisonPage(page);
+        await view.open();
+        await view.searchFor('Inter');
+
+        await expect(view.searchInput).toHaveValue('Inter');
+    });
+});

e2e/typography.test.ts

@@ -0,0 +1,44 @@
+import {
+    expect,
+    test,
+} from './fixtures';
+import type { TypographyControl } from './pages/typography-menu';
+
+/**
+ * Each control's trigger button advertises its current value via aria-label
+ * ("Size: 24"). We bump in one direction, then back, and assert the value
+ * tracks symmetrically.
+ */
+const controls: TypographyControl[] = ['size', 'weight', 'leading', 'tracking'];
+
+test.describe('typography settings', () => {
+    for (const control of controls) {
+        test(`${control}: increase then decrease returns to baseline`, async ({ typography }) => {
+            const baseline = await typography.readValue(control);
+            expect(baseline).not.toBeNull();
+
+            await typography.bump(control, 'up');
+            const bumped = await typography.readValue(control);
+            expect(bumped).not.toBe(baseline);
+            expect(bumped! > baseline!).toBe(true);
+
+            await typography.bump(control, 'down');
+            const restored = await typography.readValue(control);
+            expect(restored).toBe(baseline);
+        });
+    }
+
+    test('font size step is reflected in the persisted typography state', async ({ comparison, typography }) => {
+        await typography.bump('size', 'up');
+        const expected = await typography.readValue('size');
+
+        await expect.poll(async () => {
+            const storage = await comparison.readStorage();
+            const raw = storage['glyphdiff:comparison:typography'];
+            if (!raw) {
+                return null;
+            }
+            return JSON.parse(raw).fontSize ?? null;
+        }).toBe(expected);
+    });
+});

index.html

@@ -4,6 +4,7 @@
         <meta charset="UTF-8" />
         <meta name="viewport" content="width=device-width, initial-scale=1.0" />
         <title>glyphdiff</title>
+        <script src="https://mcp.figma.com/mcp/html-to-design/capture.js" async></script>
     </head>
     <body>
         <div id="app"></div>

lefthook.yml

@@ -13,11 +13,15 @@ pre-commit:
 pre-push:
   parallel: true
   commands:
+    test-unit:
+      run: yarn test:unit
+    test-component:
+      run: yarn test:component
     type-check:
       run: yarn tsc --noEmit
 
     svelte-check:
-      run: yarn check:shadcn-excluded --threshold warning
+      run: yarn check --threshold warning
 
     format-check:
       glob: "*.{ts,js,svelte,json,md}"

oxlint.json

@@ -1,27 +0,0 @@
-{
-  "categories": {
-    "correctness": "error",
-    "suspicious": "warn",
-    "perf": "warn",
-    "style": "warn",
-    "restriction": "error"
-  },
-  "env": {
-    "browser": true,
-    "es2021": true
-  },
-  "ignore": [
-    "node_modules",
-    "dist",
-    "build",
-    ".svelte-kit",
-    ".vercel",
-    "*.config.js",
-    "*.config.ts"
-  ],
-  "rules": {
-    "no-console": "off",
-    "no-debugger": "error",
-    "no-alert": "warn"
-  }
-}

package.json

@@ -4,6 +4,10 @@
   "version": "0.0.1",
   "packageManager": "yarn@4.11.0",
   "type": "module",
+  "sideEffects": [
+    "*.css",
+    "**/router.ts"
+  ],
   "scripts": {
     "dev": "vite",
     "build": "vite build",
@@ -11,7 +15,6 @@
     "prepare": "svelte-check --tsconfig ./tsconfig.json || echo ''",
     "check": "svelte-check",
     "check:watch": "svelte-check --tsconfig ./tsconfig.json --watch",
-    "check:shadcn-excluded": "svelte-check --no-tsconfig --ignore \"src/shared/shadcn\"",
     "lint": "oxlint",
     "format": "dprint fmt",
     "format:check": "dprint check",
@@ -28,44 +31,45 @@
     "build-storybook": "storybook build"
   },
   "devDependencies": {
-    "@chromatic-com/storybook": "^4.1.3",
-    "@internationalized/date": "^3.10.0",
-    "@lucide/svelte": "^0.561.0",
-    "@playwright/test": "^1.57.0",
-    "@storybook/addon-a11y": "^10.1.11",
-    "@storybook/addon-docs": "^10.1.11",
-    "@storybook/addon-svelte-csf": "^5.0.10",
-    "@storybook/addon-vitest": "^10.1.11",
-    "@storybook/svelte-vite": "^10.1.11",
-    "@sveltejs/vite-plugin-svelte": "^6.2.1",
-    "@tailwindcss/vite": "^4.1.18",
+    "@chromatic-com/storybook": "5.1.2",
+    "@internationalized/date": "3.12.1",
+    "@lucide/svelte": "^1.14.0",
+    "@playwright/test": "1.59.1",
+    "@storybook/addon-a11y": "10.3.6",
+    "@storybook/addon-docs": "10.3.6",
+    "@storybook/addon-svelte-csf": "5.1.2",
+    "@storybook/addon-vitest": "10.3.6",
+    "@storybook/svelte-vite": "10.3.6",
+    "@sveltejs/vite-plugin-svelte": "7.1.0",
+    "@tailwindcss/vite": "4.2.4",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/svelte": "^5.3.1",
-    "@tsconfig/svelte": "^5.0.6",
-    "@types/jsdom": "^27",
-    "@vitest/browser-playwright": "^4.0.16",
-    "@vitest/coverage-v8": "^4.0.16",
-    "bits-ui": "^2.14.4",
+    "@tsconfig/svelte": "5.0.8",
+    "@types/jsdom": "28.0.1",
+    "@vitest/browser-playwright": "4.1.5",
+    "@vitest/coverage-v8": "4.1.5",
     "clsx": "^2.1.1",
-    "dprint": "^0.50.2",
-    "jsdom": "^27.4.0",
-    "lefthook": "^2.0.13",
-    "oxlint": "^1.35.0",
-    "playwright": "^1.57.0",
-    "storybook": "^10.1.11",
-    "svelte": "^5.45.6",
-    "svelte-check": "^4.3.4",
-    "svelte-language-server": "^0.17.23",
-    "tailwind-merge": "^3.4.0",
+    "dprint": "0.54.0",
+    "jsdom": "29.1.1",
+    "lefthook": "2.1.6",
+    "oxlint": "1.62.0",
+    "playwright": "1.59.1",
+    "storybook": "10.3.6",
+    "svelte": "5.55.5",
+    "svelte-check": "4.4.8",
+    "svelte-language-server": "0.18.0",
+    "tailwind-merge": "3.5.0",
     "tailwind-variants": "^3.2.2",
-    "tailwindcss": "^4.1.18",
+    "tailwindcss": "4.2.4",
     "tw-animate-css": "^1.4.0",
-    "typescript": "^5.9.3",
-    "vite": "^7.2.6",
-    "vitest": "^4.0.16",
-    "vitest-browser-svelte": "^2.0.1"
+    "typescript": "6.0.3",
+    "vite": "8.0.10",
+    "vitest": "4.1.5",
+    "vitest-browser-svelte": "2.1.1"
   },
   "dependencies": {
-    "@tanstack/svelte-query": "^6.0.14"
+    "@chenglou/pretext": "0.0.6",
+    "@tanstack/svelte-query": "6.1.28",
+    "sv-router": "^0.16.3"
   }
 }

playwright.config.ts

@@ -1,10 +1,54 @@
-import { defineConfig } from '@playwright/test';
+import {
+    defineConfig,
+    devices,
+} from '@playwright/test';
+
+/**
+ * E2E config. Tests run against the production build via `vite preview` on port 4173.
+ * Locally: all three browser engines run in parallel.
+ * CI: chromium only, workers=1 — the runner has 6GB RAM and `yarn build` already
+ * spikes 1–2GB, so we keep the E2E peak bounded.
+ */
+const isCI = !!process.env.CI;
 
 export default defineConfig({
+    testDir: 'e2e',
+    testMatch: /.*\.test\.ts$/,
+
+    fullyParallel: true,
+    forbidOnly: isCI,
+    retries: isCI ? 2 : 0,
+    workers: isCI ? 1 : undefined,
+
+    reporter: isCI
+        ? [['html', { open: 'never' }], ['github']]
+        : [['html', { open: 'on-failure' }], ['list']],
+
+    use: {
+        baseURL: 'http://localhost:4173',
+        trace: 'on-first-retry',
+        screenshot: 'only-on-failure',
+        video: 'retain-on-failure',
+    },
+
+    projects: isCI
+        ? [{ name: 'chromium', use: { ...devices['Desktop Chrome'] } }]
+        : [
+            {
+                name: 'chromium',
+                use: {
+                    ...devices['Desktop Chrome'],
+                },
+            },
+            {
+                name: 'firefox',
+                use: { ...devices['Desktop Firefox'] },
+            },
+        ],
     webServer: {
         command: 'yarn build && yarn preview',
         port: 4173,
-        reuseExistingServer: true,
+        reuseExistingServer: !isCI,
+        timeout: 120_000,
     },
-    testDir: 'e2e',
 });

src/app/App.svelte

@@ -1,22 +1,32 @@
+<!--
+    Component: App
+    Application root with query provider and layout
+-->
 <script lang="ts">
 /**
  * App Component
  *
- * Application entry point component. Wraps the main page route within the shared
+ * Application entry point component. Wraps the active route within the shared
  * layout shell. This is the root component mounted by the application.
  *
  * Structure:
  * - QueryProvider provides TanStack Query client for data fetching
  * - Layout provides sidebar, header/footer, and page container
- * - Page renders the current route content
+ * - Router renders the matched route component
  */
-import Page from '$routes/Page.svelte';
-import { QueryProvider } from './providers';
+import '$routes/router';
+import { Router } from 'sv-router';
+import {
+    AppBindingsProvider,
+    QueryProvider,
+} from './providers';
 import Layout from './ui/Layout.svelte';
 </script>
 
 <QueryProvider>
+    <AppBindingsProvider>
         <Layout>
-        <Page />
+            <Router />
         </Layout>
+    </AppBindingsProvider>
 </QueryProvider>

src/app/providers/AppBindings.svelte

@@ -0,0 +1,24 @@
+<!--
+    Component: AppBindings
+    Provider that starts app-wide store bindings (filters → sort → font catalog)
+    for its subtree. Mount-scoped so the bindings' lifetime tracks the app tree.
+-->
+<script lang="ts">
+import { startFilterBindings } from '$features/FilterAndSortFonts';
+import { onMount } from 'svelte';
+import type { Snippet } from 'svelte';
+
+interface Props {
+    /**
+     * Content snippet
+     */
+    children?: Snippet;
+}
+
+let { children }: Props = $props();
+
+// startFilterBindings returns its $effect.root cleanup; onMount runs it on unmount.
+onMount(() => startFilterBindings());
+</script>
+
+{@render children?.()}

src/app/providers/QueryProvider.svelte

@@ -1,15 +1,26 @@
-<script lang="ts">
-/**
- * Query Provider Component
- *
- * All components that use useQueryClient() or createQuery() must be
- * descendants of this provider.
- */
-import { queryClient } from '$shared/api/queryClient';
-import { QueryClientProvider } from '@tanstack/svelte-query';
+<!--
+    Component: QueryProvider
+    Provides a QueryClientProvider for child components.
 
-/** Slot content for child components */
-let { children } = $props();
+    All components that use useQueryClient() or createQuery() must be
+    descendants of this provider.
+-->
+<script lang="ts">
+import { getQueryClient } from '$shared/api/queryClient';
+import { QueryClientProvider } from '@tanstack/svelte-query';
+import type { Snippet } from 'svelte';
+
+interface Props {
+    /**
+     * Content snippet
+     */
+    children?: Snippet;
+}
+
+let { children }: Props = $props();
+
+// First call to the lazy singleton — constructs the shared client for the app.
+const queryClient = getQueryClient();
 </script>
 
 <QueryClientProvider client={queryClient}>

src/app/providers/index.ts

@@ -1 +1,2 @@
+export { default as AppBindingsProvider } from './AppBindings.svelte';
 export { default as QueryProvider } from './QueryProvider.svelte';

src/app/styles/app.css

@@ -1,83 +1,160 @@
 @import "tailwindcss";
-
 @import "tw-animate-css";
+@import "./fonts.css";
 
-@custom-variant dark (&:is(.dark *));
+@variant dark (&:where(.dark, .dark *));
 
 :root {
-  --radius: 0.625rem;
-  --background: oklch(1 0 0);
-  --foreground: oklch(0.141 0.005 285.823);
-  --card: oklch(1 0 0);
-  --card-foreground: oklch(0.141 0.005 285.823);
+  /* Base font size */
+  --font-size: 16px;
+
+  /* GLYPHDIFF Design System */
+  /* Primary Colors */
+  --swiss-beige: #f3f0e9;
+  --swiss-red: #ff3b30;
+  --swiss-black: #1a1a1a;
+  --swiss-white: #ffffff;
+
+  /* Semantic mode-switching colors. These are redefined inside `.dark`
+     so utilities that reference them auto-adapt without a `dark:` variant. */
+  --color-border-subtle: var(--neutral-300);
+  --color-text-subtle: var(--neutral-500);
+  --color-skeleton: var(--neutral-200);
+  --color-grid-line: rgb(0 0 0 / 0.03);
+
+  /* Neutral Grays */
+  --neutral-50: #fafafa;
+  --neutral-100: #f5f5f5;
+  --neutral-200: #e5e5e5;
+  --neutral-300: #d4d4d4;
+  --neutral-400: #a3a3a3;
+  --neutral-500: #737373;
+  --neutral-600: #525252;
+  --neutral-700: #404040;
+  --neutral-800: #262626;
+  --neutral-900: #171717;
+
+  /* Dark Mode Backgrounds */
+  --dark-bg: #121212;
+  --dark-card: #1e1e1e;
+  --dark-border: rgba(255, 255, 255, 0.1);
+
+  /* Light Mode Backgrounds */
+  --light-bg: #f3f0e9;
+  --light-card: #ffffff;
+  --light-border: rgba(0, 0, 0, 0.05);
+
+  /* Semantic Colors */
+  --color-brand: var(--swiss-red);
+  --color-surface: var(--swiss-beige);
+  --color-paper: var(--swiss-white);
+
+  /* Base Tailwind Colors (for compatibility) */
+  --background: #ffffff;
+  --foreground: oklch(0.145 0 0);
+  --card: #ffffff;
+  --card-foreground: oklch(0.145 0 0);
   --popover: oklch(1 0 0);
-  --popover-foreground: oklch(0.141 0.005 285.823);
-  --primary: oklch(0.21 0.006 285.885);
-  --primary-foreground: oklch(0.985 0 0);
-  --secondary: oklch(0.967 0.001 286.375);
-  --secondary-foreground: oklch(0.21 0.006 285.885);
-  --muted: oklch(0.967 0.001 286.375);
-  --muted-foreground: oklch(0.552 0.016 285.938);
-  --accent: oklch(0.967 0.001 286.375);
-  --accent-foreground: oklch(0.21 0.006 285.885);
-  --destructive: oklch(0.577 0.245 27.325);
-  --border: oklch(0.92 0.004 286.32);
-  --input: oklch(0.92 0.004 286.32);
-  --ring: oklch(0.705 0.015 286.067);
+  --popover-foreground: oklch(0.145 0 0);
+  --primary: #030213;
+  --primary-foreground: oklch(1 0 0);
+  --secondary: oklch(0.95 0.0058 264.53);
+  --secondary-foreground: #030213;
+  --muted: #ececf0;
+  --muted-foreground: #717182;
+  --accent: #e9ebef;
+  --accent-foreground: #030213;
+  --destructive: #d4183d;
+  --destructive-foreground: #ffffff;
+  --border: rgba(0, 0, 0, 0.1);
+  --input: transparent;
+  --input-background: #f3f3f5;
+  --switch-background: #cbced4;
+  --font-weight-medium: 500;
+  --font-weight-normal: 400;
+  --ring: oklch(0.708 0 0);
   --chart-1: oklch(0.646 0.222 41.116);
   --chart-2: oklch(0.6 0.118 184.704);
   --chart-3: oklch(0.398 0.07 227.392);
   --chart-4: oklch(0.828 0.189 84.429);
   --chart-5: oklch(0.769 0.188 70.08);
+  --radius: 0rem;
   --sidebar: oklch(0.985 0 0);
-  --sidebar-foreground: oklch(0.141 0.005 285.823);
-  --sidebar-primary: oklch(0.21 0.006 285.885);
+  --sidebar-foreground: oklch(0.145 0 0);
+  --sidebar-primary: #030213;
   --sidebar-primary-foreground: oklch(0.985 0 0);
-  --sidebar-accent: oklch(0.967 0.001 286.375);
-  --sidebar-accent-foreground: oklch(0.21 0.006 285.885);
-  --sidebar-border: oklch(0.92 0.004 286.32);
-  --sidebar-ring: oklch(0.705 0.015 286.067);
+  --sidebar-accent: oklch(0.97 0 0);
+  --sidebar-accent-foreground: oklch(0.205 0 0);
+  --sidebar-border: oklch(0.922 0 0);
+  --sidebar-ring: oklch(0.708 0 0);
+
+  /* Typography Scale */
+  --text-xs: 0.75rem;
+  --text-sm: 0.875rem;
+  --text-base: 1rem;
+  --text-lg: 1.125rem;
+  --text-xl: 1.25rem;
+  --text-2xl: 1.5rem;
+  --text-3xl: 1.875rem;
+  --text-4xl: 2.25rem;
+  --text-5xl: 3rem;
+  --text-6xl: 3.75rem;
+  --text-7xl: 4.5rem;
+  --text-8xl: 6rem;
+
+  /* Comparison Font Sizes */
+  --comparison-font-mobile: 3rem;
+  --comparison-font-tablet: 4.5rem;
+  --comparison-font-desktop: 6rem;
 }
 
 .dark {
-  --background: oklch(0.141 0.005 285.823);
+  --color-surface: var(--dark-bg);
+  --color-paper: var(--dark-card);
+
+  /* Dark-mode overrides for the semantic mode-switching colors. */
+  --color-border-subtle: rgb(255 255 255 / 0.1);
+  --color-text-subtle: var(--neutral-400);
+  --color-skeleton: var(--neutral-800);
+  --color-grid-line: rgb(255 255 255 / 0.05);
+
+  --background: oklch(0.145 0 0);
   --foreground: oklch(0.985 0 0);
-  --card: oklch(0.21 0.006 285.885);
+  --card: oklch(0.145 0 0);
   --card-foreground: oklch(0.985 0 0);
-  --popover: oklch(0.21 0.006 285.885);
+  --popover: oklch(0.145 0 0);
   --popover-foreground: oklch(0.985 0 0);
-  --primary: oklch(0.92 0.004 286.32);
-  --primary-foreground: oklch(0.21 0.006 285.885);
-  --secondary: oklch(0.274 0.006 286.033);
+  --primary: oklch(0.985 0 0);
+  --primary-foreground: oklch(0.205 0 0);
+  --secondary: oklch(0.269 0 0);
   --secondary-foreground: oklch(0.985 0 0);
-  --muted: oklch(0.274 0.006 286.033);
-  --muted-foreground: oklch(0.705 0.015 286.067);
-  --accent: oklch(0.274 0.006 286.033);
+  --muted: oklch(0.269 0 0);
+  --muted-foreground: oklch(0.708 0 0);
+  --accent: oklch(0.269 0 0);
   --accent-foreground: oklch(0.985 0 0);
-  --destructive: oklch(0.704 0.191 22.216);
-  --border: oklch(1 0 0 / 10%);
-  --input: oklch(1 0 0 / 15%);
-  --ring: oklch(0.552 0.016 285.938);
+  --destructive: oklch(0.396 0.141 25.723);
+  --destructive-foreground: oklch(0.637 0.237 25.331);
+  --border: oklch(0.269 0 0);
+  --input: oklch(0.269 0 0);
+  --ring: oklch(0.439 0 0);
+  --font-weight-medium: 500;
+  --font-weight-normal: 400;
   --chart-1: oklch(0.488 0.243 264.376);
   --chart-2: oklch(0.696 0.17 162.48);
   --chart-3: oklch(0.769 0.188 70.08);
   --chart-4: oklch(0.627 0.265 303.9);
   --chart-5: oklch(0.645 0.246 16.439);
-  --sidebar: oklch(0.21 0.006 285.885);
+  --sidebar: oklch(0.205 0 0);
   --sidebar-foreground: oklch(0.985 0 0);
   --sidebar-primary: oklch(0.488 0.243 264.376);
   --sidebar-primary-foreground: oklch(0.985 0 0);
-  --sidebar-accent: oklch(0.274 0.006 286.033);
+  --sidebar-accent: oklch(0.269 0 0);
   --sidebar-accent-foreground: oklch(0.985 0 0);
-  --sidebar-border: oklch(1 0 0 / 10%);
-  --sidebar-ring: oklch(0.552 0.016 285.938);
+  --sidebar-border: oklch(0.269 0 0);
+  --sidebar-ring: oklch(0.439 0 0);
 }
 
-@theme inline {
-  --radius-sm: calc(var(--radius) - 4px);
-  --radius-md: calc(var(--radius) - 2px);
-  --radius-lg: var(--radius);
-  --radius-xl: calc(var(--radius) + 4px);
+@theme {
   --color-background: var(--background);
   --color-foreground: var(--foreground);
   --color-card: var(--card);
@@ -93,14 +170,21 @@
   --color-accent: var(--accent);
   --color-accent-foreground: var(--accent-foreground);
   --color-destructive: var(--destructive);
+  --color-destructive-foreground: var(--destructive-foreground);
   --color-border: var(--border);
   --color-input: var(--input);
+  --color-input-background: var(--input-background);
+  --color-switch-background: var(--switch-background);
   --color-ring: var(--ring);
   --color-chart-1: var(--chart-1);
   --color-chart-2: var(--chart-2);
   --color-chart-3: var(--chart-3);
   --color-chart-4: var(--chart-4);
   --color-chart-5: var(--chart-5);
+  --radius-sm: 0rem;
+  --radius-md: 0rem;
+  --radius-lg: 0rem;
+  --radius-xl: 0rem;
   --color-sidebar: var(--sidebar);
   --color-sidebar-foreground: var(--sidebar-foreground);
   --color-sidebar-primary: var(--sidebar-primary);
@@ -109,20 +193,240 @@
   --color-sidebar-accent-foreground: var(--sidebar-accent-foreground);
   --color-sidebar-border: var(--sidebar-border);
   --color-sidebar-ring: var(--sidebar-ring);
+
+  --color-swiss-beige: var(--swiss-beige);
+  --color-swiss-red: var(--swiss-red);
+  --color-swiss-black: var(--swiss-black);
+  --color-swiss-white: var(--swiss-white);
+  --color-brand: var(--color-brand);
+  --color-surface: var(--color-surface);
+  --color-paper: var(--color-paper);
+  --color-dark-bg: var(--dark-bg);
+  --color-dark-card: var(--dark-card);
+
+  --font-logo: 'Syne', system-ui, -apple-system, 'Segoe UI', Inter, Roboto, Arial, sans-serif;
+  --font-mono: 'Space Mono', monospace;
+  --font-primary: 'Space Grotesk', system-ui, -apple-system, 'Segoe UI', Inter, Roboto, Arial, sans-serif;
+  --font-secondary: 'Inter', system-ui, -apple-system, 'Segoe UI', Roboto, Arial, sans-serif;
+
+  /* Micro typography scale — extends Tailwind's text-xs (0.75rem) downward */
+  --text-5xs: 0.4375rem;
+  --text-4xs: 0.5rem;
+  --text-3xs: 0.5625rem;
+  --text-2xs: 0.625rem;
+  /* Monospace label tracking — used in Loader and Footnote */
+  --tracking-wider-mono: 0.2em;
+
+  /* Shadow tokens */
+
+  /* Default resting shadow — equivalent to Tailwind's shadow-sm. Used on
+     buttons, sliders, popover triggers in non-floating state. */
+  --shadow-rest: 0 1px 2px 0 rgb(0 0 0 / 0.05);
+
+  /* Swiss "hard offset" stamp — rests at 2px/2px, lifts to 3px/3px on
+     hover, presses back to 1px/1px on active. Primary button motif. */
+  --shadow-stamp-rest: 0.125rem 0.125rem 0 0 rgb(0 0 0 / 0.1);
+  --shadow-stamp-hover: 0.1875rem 0.1875rem 0 0 rgb(0 0 0 / 0.15);
+  --shadow-stamp-pressed: 0.0625rem 0.0625rem 0 0 rgb(0 0 0 / 0.1);
+
+  /* Card-tier hard-offset stamp — wider, brand-tinted. Used on
+     interactive cards (FontSampler hover). */
+  --shadow-stamp-card: 5px 5px 0 0 var(--color-brand);
+
+  /* Floating popovers (typography menu, combo control list). */
+  --shadow-popover: 0 20px 40px -10px rgb(0 0 0 / 0.15);
+
+  /* Drop-shadow under semi-translucent floating panels like the
+     comparison slider's character row. */
+  --shadow-floating-panel: 0 25px 50px -12px rgb(0 0 0 / 0.05);
+  --shadow-floating-panel-dark: 0 25px 50px -12px rgb(0 0 0 / 0.2);
+
+  /* Drawer / overlay shadow — full-strength shadow-2xl. */
+  --shadow-overlay: 0 25px 50px -12px rgb(0 0 0 / 0.25);
+
+  /* Motion tokens */
+
+  --duration-fast: 150ms;
+  --duration-normal: 200ms;
+  --duration-slow: 300ms;
+  --duration-slower: 500ms;
+
+  /* Tailwind's default ease-in-out — symmetric, good for layout shifts. */
+  --ease-standard: cubic-bezier(0.4, 0, 0.2, 1);
+  /* Decelerating curve — matches Tailwind's ease-out. Dominant in this codebase. */
+  --ease-out-soft: cubic-bezier(0, 0, 0.2, 1);
+  /* Spring overshoot — used in character pop animation. */
+  --ease-spring-overshoot: cubic-bezier(0.34, 1.56, 0.64, 1);
 }
 
 @layer base {
   * {
     @apply border-border outline-ring/50;
   }
+
+  ::selection {
+    background-color: var(--color-brand);
+    color: var(--swiss-white);
+  }
+
   body {
     @apply bg-background text-foreground;
-    font-family: 'Karla', system-ui, sans-serif;
+    font-family: var(--font-secondary);
     font-optical-sizing: auto;
   }
+
+  html {
+    font-size: var(--font-size);
+  }
+
+  h1 {
+    font-size: var(--text-2xl);
+    font-weight: var(--font-weight-medium);
+    line-height: 1.5;
+  }
+
+  h2 {
+    font-size: var(--text-xl);
+    font-weight: var(--font-weight-medium);
+    line-height: 1.5;
+  }
+
+  h3 {
+    font-size: var(--text-lg);
+    font-weight: var(--font-weight-medium);
+    line-height: 1.5;
+  }
+
+  h4 {
+    font-size: var(--text-base);
+    font-weight: var(--font-weight-medium);
+    line-height: 1.5;
+  }
+
+  label {
+    font-size: var(--text-base);
+    font-weight: var(--font-weight-medium);
+    line-height: 1.5;
+  }
+
+  button {
+    font-size: var(--text-base);
+    font-weight: var(--font-weight-medium);
+    line-height: 1.5;
+  }
+
+  input {
+    font-size: var(--text-base);
+    font-weight: var(--font-weight-normal);
+    line-height: 1.5;
+  }
 }
 
-/* Global utility - useful across your app */
+/* Design-system utilities.
+   Defined via `@utility` (Tailwind v4) so they integrate with the variant
+   system (`hover:`, `dark:`, breakpoints) and don't rely on `@apply`
+   chains. Colors reference the mode-switching semantic vars defined in
+   `:root`/`.dark` above, so most utilities need no `dark:` variant in
+   their definition or at call sites. */
+
+@utility border-subtle {
+  border-color: var(--color-border-subtle);
+}
+
+/* Same color as border-subtle, applied via background-color — for 1px
+   dividers, inline separator strips, and other hairlines that aren't
+   element borders. */
+@utility bg-subtle {
+  background-color: var(--color-border-subtle);
+}
+
+/* Muted text color — paired with `border-subtle` naming. The previous
+   name `text-secondary` collided with Tailwind v4 auto-generating a
+   utility from `--color-secondary` (the shadcn near-white surface token
+   registered in `@theme`), which made every consumer effectively
+   invisible (near-white text on light backgrounds). */
+@utility text-subtle {
+  color: var(--color-text-subtle);
+}
+
+@utility focus-ring {
+  &:focus-visible {
+    outline: 2px solid transparent;
+    outline-offset: 2px;
+    box-shadow: 0 0 0 2px var(--color-background, white), 0 0 0 4px var(--color-brand);
+  }
+}
+
+/* Surface utilities */
+
+@utility surface-canvas {
+  background-color: var(--color-surface);
+}
+
+@utility surface-card {
+  background-color: var(--color-paper);
+  border: 1px solid var(--color-border-subtle);
+}
+
+@utility surface-card-elevated {
+  background-color: var(--color-paper);
+  border: 1px solid var(--color-border-subtle);
+  box-shadow: var(--shadow-rest);
+}
+
+@utility surface-popover {
+  background-color: var(--color-paper);
+  border: 1px solid var(--color-border-subtle);
+  box-shadow: var(--shadow-popover);
+}
+
+@utility surface-floating {
+  background-color: color-mix(in srgb, var(--color-surface) 80%, transparent);
+  backdrop-filter: blur(12px);
+  border: 1px solid var(--color-border-subtle);
+}
+
+/* Shape / layout */
+
+@utility flex-center {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+
+@utility skeleton-fill {
+  background-color: color-mix(in srgb, var(--color-skeleton) 70%, transparent);
+}
+
+/* Subtle dotted-grid overlay used as a decorative background on the
+   comparison paper surface. Color and intensity auto-switch via
+   --color-grid-line. `bg-grid-sm` uses a tighter cell — typical mobile
+   choice; `bg-grid` is the default desktop cell. Pair with absolute /
+   pointer-events-none on the overlay element. */
+@utility bg-grid {
+  background-image:
+    linear-gradient(var(--color-grid-line) 1px, transparent 1px),
+    linear-gradient(90deg, var(--color-grid-line) 1px, transparent 1px);
+  background-size: 20px 20px;
+}
+
+@utility bg-grid-sm {
+  background-image:
+    linear-gradient(var(--color-grid-line) 1px, transparent 1px),
+    linear-gradient(90deg, var(--color-grid-line) 1px, transparent 1px);
+  background-size: 10px 10px;
+}
+
+/* Typography */
+
+@utility text-label-mono {
+  font-family: var(--font-primary);
+  font-weight: 700;
+  letter-spacing: -0.025em;
+  text-transform: uppercase;
+}
+
+/* Honor prefers-reduced-motion: collapse animation and transition timing. */
 @media (prefers-reduced-motion: reduce) {
     * {
         animation-duration: 0.01ms !important;
@@ -131,12 +435,12 @@
     }
 }
 
-/* Performance optimization for collapsible elements */
+/* Hint the upcoming height animation on open collapsibles. */
 [data-state="open"] {
     will-change: height;
 }
 
-/* Smooth focus transitions - good globally */
+/* Transition siblings of a focus-visible peer. */
 .peer:focus-visible ~ * {
     transition: all 150ms cubic-bezier(0.4, 0, 0.2, 1);
 }
@@ -162,3 +466,71 @@
 .animate-nudge {
     animation: nudge 10s ease-in-out infinite;
 }
+
+/* Scrollbar styling */
+
+/* Standard API: color + width (Chrome 121+, Firefox 64+). */
+@supports (scrollbar-width: auto) {
+    * {
+        scrollbar-width: thin;
+        scrollbar-color: hsl(0 0% 70% / 0.4) var(--color-surface);
+    }
+
+    .dark * {
+        scrollbar-color: hsl(0 0% 40% / 0.5) var(--color-surface);
+    }
+}
+
+/* WebKit fallback: applies on top of the standard API in Chrome, standalone in
+   older Safari. Covers what scrollbar-width can't — hiding buttons, exact sizing. */
+@supports selector(::-webkit-scrollbar) {
+    ::-webkit-scrollbar {
+        width: 6px;
+        height: 6px;
+    }
+
+    ::-webkit-scrollbar-button {
+        display: none; /* hide scrollbar buttons */
+    }
+
+    ::-webkit-scrollbar-track {
+        background: var(--color-surface);
+    }
+
+    ::-webkit-scrollbar-thumb {
+        background: hsl(0 0% 70% / 0.4);
+        border-radius: 3px;
+    }
+
+    ::-webkit-scrollbar-thumb:hover {
+        background: hsl(0 0% 50% / 0.6);
+    }
+
+    ::-webkit-scrollbar-thumb:active {
+        background: hsl(0 0% 40% / 0.8);
+    }
+
+    ::-webkit-scrollbar-corner {
+        background: var(--color-surface);
+    }
+
+    .dark ::-webkit-scrollbar-thumb        { background: hsl(0 0% 40% / 0.5); }
+    .dark ::-webkit-scrollbar-thumb:hover  { background: hsl(0 0% 55% / 0.6); }
+    .dark ::-webkit-scrollbar-thumb:active { background: hsl(0 0% 65% / 0.7); }
+}
+
+html {
+    scroll-behavior: smooth;
+}
+
+@media (prefers-reduced-motion: reduce) {
+    html { scroll-behavior: auto; }
+}
+
+body {
+    overscroll-behavior-y: none;
+}
+
+.scroll-stable {
+    scrollbar-gutter: stable;
+}

src/app/styles/fonts.css

@@ -0,0 +1,78 @@
+/*
+    Self-hosted interface fonts (latin subset only).
+    Vendored from @fontsource — see docs/interface-font-selfhost-benchmark.md.
+    Variable faces (Inter, Space Grotesk) keep their wght axis; Inter also keeps opsz.
+    url()s are resolved + content-hashed by Vite at build → immutable long-cache.
+*/
+
+/* Inter — variable wght + opsz, the body/secondary UI font (--font-secondary) */
+@font-face {
+    font-family: 'Inter';
+    font-style: normal;
+    font-display: swap;
+    font-weight: 100 900;
+    src: url('../assets/fonts/inter-latin-opsz-normal.woff2') format('woff2');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+@font-face {
+    font-family: 'Inter';
+    font-style: italic;
+    font-display: swap;
+    font-weight: 100 900;
+    src: url('../assets/fonts/inter-latin-opsz-italic.woff2') format('woff2');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+
+/* Space Grotesk — variable wght, the primary/display UI font (--font-primary) */
+@font-face {
+    font-family: 'Space Grotesk';
+    font-style: normal;
+    font-display: swap;
+    font-weight: 300 700;
+    src: url('../assets/fonts/space-grotesk-latin-wght-normal.woff2') format('woff2');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+
+/* Space Mono — static 400/700 × roman/italic (--font-mono) */
+@font-face {
+    font-family: 'Space Mono';
+    font-style: normal;
+    font-display: swap;
+    font-weight: 400;
+    src: url('../assets/fonts/space-mono-latin-400-normal.woff2') format('woff2');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+@font-face {
+    font-family: 'Space Mono';
+    font-style: italic;
+    font-display: swap;
+    font-weight: 400;
+    src: url('../assets/fonts/space-mono-latin-400-italic.woff2') format('woff2');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+@font-face {
+    font-family: 'Space Mono';
+    font-style: normal;
+    font-display: swap;
+    font-weight: 700;
+    src: url('../assets/fonts/space-mono-latin-700-normal.woff2') format('woff2');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+@font-face {
+    font-family: 'Space Mono';
+    font-style: italic;
+    font-display: swap;
+    font-weight: 700;
+    src: url('../assets/fonts/space-mono-latin-700-italic.woff2') format('woff2');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+
+/* Syne — static 800, the logo font (--font-logo) */
+@font-face {
+    font-family: 'Syne';
+    font-style: normal;
+    font-display: swap;
+    font-weight: 800;
+    src: url('../assets/fonts/syne-latin-800-normal.woff2') format('woff2');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}

src/app/types/ambient.d.ts

@@ -35,3 +35,23 @@ declare module '*.jpg' {
     const content: string;
     export default content;
 }
+
+declare module '*.css';
+
+declare module '*.woff2?url' {
+    const content: string;
+    export default content;
+}
+
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+    readonly DEV: boolean;
+    readonly PROD: boolean;
+    readonly MODE: string;
+    // Add other env variables you use
+}
+
+interface ImportMeta {
+    readonly env: ImportMetaEnv;
+}

src/app/ui/Layout.svelte

@@ -1,59 +1,82 @@
+<!--
+    Component: Layout
+    Application shell with providers and page wrapper
+-->
 <script lang="ts">
-/**
- * Layout Component
- *
- * Root layout wrapper that provides the application shell structure. Handles favicon,
- * toolbar provider initialization, and renders child routes with consistent structure.
- *
- * Layout structure:
- * - Header area (currently empty, reserved for future use)
- *
- * - Footer area (currently empty, reserved for future use)
- */
-import { BreadcrumbHeader } from '$entities/Breadcrumb';
-import favicon from '$shared/assets/favicon.svg';
-import { ScrollArea } from '$shared/shadcn/ui/scroll-area';
-import { Provider as TooltipProvider } from '$shared/shadcn/ui/tooltip';
-import { TypographyMenu } from '$widgets/TypographySettings';
-import type { Snippet } from 'svelte';
+import { getThemeManager } from '$features/ChangeAppTheme';
+import G from '$shared/assets/G.svg';
+import { ResponsiveProvider } from '$shared/lib';
+import { cn } from '$shared/lib';
+import { Footer } from '$widgets/Footer';
+
+/*
+    Preload the two render-critical interface faces (primary + secondary).
+    `?url` resolves to the content-hashed path Vite emits, so the binary is
+    fetched immediately rather than waiting for CSS @font-face discovery.
+*/
+import interWoff2 from '../assets/fonts/inter-latin-opsz-normal.woff2?url';
+import spaceGroteskWoff2 from '../assets/fonts/space-grotesk-latin-wght-normal.woff2?url';
+
+import {
+    type Snippet,
+    onDestroy,
+    onMount,
+} from 'svelte';
 
 interface Props {
+    /**
+     * Content snippet
+     */
     children: Snippet;
 }
 
-/** Slot content for route pages to render */
 let { children }: Props = $props();
+let fontsReady = $state(true);
+
+const themeManager = getThemeManager();
+const theme = $derived(themeManager.value);
+
+onMount(() => themeManager.init());
+onDestroy(() => themeManager.destroy());
 </script>
 
 <svelte:head>
-    <link rel="icon" href={favicon} />
-    <link rel="preconnect" href="https://api.fontshare.com" />
-    <link rel="preconnect" href="https://cdn.fontshare.com" crossorigin="anonymous" />
+    <link rel="icon" href={G} type="image/svg+xml" />
 
-    <link rel="preconnect" href="https://fonts.googleapis.com">
-    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin="anonymous">
+    <!-- Self-hosted interface fonts (see src/app/styles/fonts/fonts.css). Preload the two critical faces. -->
     <link
-        href="https://fonts.googleapis.com/css2?family=Karla:ital,wght@0,200..800;1,200..800&display=swap"
-        rel="stylesheet"
-    >
+        rel="preload"
+        as="font"
+        type="font/woff2"
+        href={interWoff2}
+        crossorigin="anonymous"
+    />
     <link
-        href="https://fonts.googleapis.com/css2?family=Barlow:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;0,800;0,900;1,100;1,200;1,300;1,400;1,500;1,600;1,700;1,800;1,900&family=Karla:wght@300&display=swap"
-        rel="stylesheet"
-    >
+        rel="preload"
+        as="font"
+        type="font/woff2"
+        href={spaceGroteskWoff2}
+        crossorigin="anonymous"
+    />
+    <title>GlyphDiff | Typography & Typefaces</title>
+    <meta
+        name="description"
+        content="Compare typefaces side by side. Adjust size, weight, leading, and tracking to find the perfect typographic pairing."
+    />
 </svelte:head>
 
-<div id="app-root" class="min-h-screen flex flex-col bg-background">
-    <header>
-        <BreadcrumbHeader />
-    </header>
-
-    <!-- <ScrollArea class="h-screen w-screen"> -->
-    <main class="flex-1 h-full w-full max-w-6xl mx-auto px-4 pt-6 pb-10 md:px-8 lg:pt-10 lg:pb-20 relative">
-        <TooltipProvider>
-            <TypographyMenu />
+<ResponsiveProvider>
+    <div
+        id="app-root"
+        class={cn(
+            'min-h-dvh w-auto flex flex-col surface-canvas relative',
+            theme === 'dark' ? 'dark' : '',
+        )}
+    >
+        {#if fontsReady}
             {@render children?.()}
-        </TooltipProvider>
-    </main>
-    <!-- </ScrollArea> -->
-    <footer></footer>
-</div>
+        {/if}
+
+        <Footer />
+    </div>
+</ResponsiveProvider>

src/entities/Breadcrumb/index.ts

@@ -1,2 +0,0 @@
-export { scrollBreadcrumbsStore } from './model';
-export { BreadcrumbHeader } from './ui';

src/entities/Breadcrumb/model/index.ts

@@ -1 +0,0 @@
-export * from './store/scrollBreadcrumbsStore.svelte';

src/entities/Breadcrumb/model/store/scrollBreadcrumbsStore.svelte.ts

@@ -1,29 +0,0 @@
-import type { Snippet } from 'svelte';
-
-export interface BreadcrumbItem {
-    index: number;
-    title: Snippet<[{ className?: string }]>;
-}
-
-class ScrollBreadcrumbsStore {
-    #items = $state<BreadcrumbItem[]>([]);
-
-    get items() {
-        // Keep them sorted by index for Swiss orderliness
-        return this.#items.sort((a, b) => a.index - b.index);
-    }
-    add(item: BreadcrumbItem) {
-        if (!this.#items.find(i => i.index === item.index)) {
-            this.#items.push(item);
-        }
-    }
-    remove(index: number) {
-        this.#items = this.#items.filter(i => i.index !== index);
-    }
-}
-
-export function createScrollBreadcrumbsStore() {
-    return new ScrollBreadcrumbsStore();
-}
-
-export const scrollBreadcrumbsStore = createScrollBreadcrumbsStore();

src/entities/Breadcrumb/ui/BreadcrumbHeader/BreadcrumbHeader.svelte

@@ -1,77 +0,0 @@
-<!--
-    Component: BreadcrumbHeader
-    Fixed header for breadcrumbs navigation for sections in the page
--->
-<script lang="ts">
-import { cn } from '$shared/shadcn/utils/shadcn-utils';
-import {
-    fly,
-    slide,
-} from 'svelte/transition';
-import { scrollBreadcrumbsStore } from '../../model';
-</script>
-
-{#if scrollBreadcrumbsStore.items.length > 0}
-    <div
-        transition:slide={{ duration: 200 }}
-        class="
-            fixed top-0 left-0 right-0 z-100
-            backdrop-blur-lg bg-white/20
-            border-b border-gray-300/50
-            shadow-[0_1px_3px_rgba(0,0,0,0.04)]
-            h-12
-        "
-    >
-        <div class="max-w-8xl mx-auto px-6 h-full flex items-center gap-4">
-            <h1 class={cn('font-[Barlow] font-extralight')}>
-                GLYPHDIFF
-            </h1>
-
-            <div class="h-4 w-px bg-gray-300/60"></div>
-
-            <nav class="flex items-center gap-3 overflow-x-auto scrollbar-hide flex-1">
-                {#each scrollBreadcrumbsStore.items as item, idx (item.index)}
-                    <div
-                        in:fly={{ duration: 300, y: -10, x: 100, opacity: 0 }}
-                        out:fly={{ duration: 300, y: 10, x: 100, opacity: 0 }}
-                        class="flex items-center gap-3 whitespace-nowrap shrink-0"
-                    >
-                        <span class="font-mono text-[9px] text-gray-400 tracking-wider">
-                            {String(item.index).padStart(2, '0')}
-                        </span>
-
-                        {@render item.title({
-    className: 'text-[10px] md:text-[10px] font-bold uppercase tracking-tight leading-[0.95] text-gray-900',
-})}
-
-                        {#if idx < scrollBreadcrumbsStore.items.length - 1}
-                            <div class="flex items-center gap-0.5 opacity-40">
-                                <div class="w-1 h-px bg-gray-400"></div>
-                                <div class="w-1 h-px bg-gray-400"></div>
-                                <div class="w-1 h-px bg-gray-400"></div>
-                            </div>
-                        {/if}
-                    </div>
-                {/each}
-            </nav>
-
-            <div class="flex items-center gap-2 opacity-50 ml-auto">
-                <div class="w-px h-2.5 bg-gray-300/60"></div>
-                <span class="font-mono text-[8px] text-gray-400 tracking-wider">
-                    [{scrollBreadcrumbsStore.items.length}]
-                </span>
-            </div>
-        </div>
-    </div>
-{/if}
-
-<style>
-/* Hide scrollbar but keep functionality */
-.scrollbar-hide {
-    -ms-overflow-style: none;
-    scrollbar-width: none;
-}
-.scrollbar-hide::-webkit-scrollbar {
-    display: none;
-}
-</style>

src/entities/Breadcrumb/ui/index.ts

@@ -1,3 +0,0 @@
-import BreadcrumbHeader from './BreadcrumbHeader/BreadcrumbHeader.svelte';
-
-export { BreadcrumbHeader };

src/entities/Font/api/fontshare/fontshare.ts

@@ -1,161 +0,0 @@
-/**
- * Fontshare API client
- *
- * Handles API requests to Fontshare API for fetching font metadata.
- * Provides error handling, pagination support, and type-safe responses.
- *
- * Pagination: The Fontshare API DOES support pagination via `page` and `limit` parameters.
- * However, the current implementation uses `fetchAllFontshareFonts()` to fetch all fonts upfront.
- * For future optimization, consider implementing incremental pagination for large datasets.
- *
- * @see https://fontshare.com
- */
-
-import { api } from '$shared/api/api';
-import { buildQueryString } from '$shared/lib/utils';
-import type { QueryParams } from '$shared/lib/utils';
-import type {
-    FontshareApiModel,
-    FontshareFont,
-} from '../../model/types/fontshare';
-
-/**
- * Fontshare API parameters
- */
-export interface FontshareParams extends QueryParams {
-    /**
-     * Filter by categories (e.g., ["Sans", "Serif", "Display"])
-     */
-    categories?: string[];
-    /**
-     * Filter by tags (e.g., ["Magazines", "Branding", "Logos"])
-     */
-    tags?: string[];
-    /**
-     * Page number for pagination (1-indexed)
-     */
-    page?: number;
-    /**
-     * Number of items per page
-     */
-    limit?: number;
-    /**
-     * Search query to filter fonts
-     */
-    q?: string;
-}
-
-/**
- * Fontshare API response wrapper
- * Re-exported from model/types/fontshare for backward compatibility
- */
-export type FontshareResponse = FontshareApiModel;
-
-/**
- * Fetch fonts from Fontshare API
- *
- * @param params - Query parameters for filtering fonts
- * @returns Promise resolving to Fontshare API response
- * @throws ApiError when request fails
- *
- * @example
- * ```ts
- * // Fetch all Sans category fonts
- * const response = await fetchFontshareFonts({
- *     categories: ['Sans'],
- *     limit: 50
- * });
- *
- * // Fetch fonts with specific tags
- * const response = await fetchFontshareFonts({
- *     tags: ['Branding', 'Logos']
- * });
- *
- * // Search fonts
- * const response = await fetchFontshareFonts({
- *     search: 'Satoshi'
- * });
- * ```
- */
-export async function fetchFontshareFonts(
-    params: FontshareParams = {},
-): Promise<FontshareResponse> {
-    const queryString = buildQueryString(params);
-    const url = `https://api.fontshare.com/v2/fonts${queryString}`;
-
-    try {
-        const response = await api.get<FontshareResponse>(url);
-        return response.data;
-    } catch (error) {
-        // Re-throw ApiError with context
-        if (error instanceof Error) {
-            throw error;
-        }
-        throw new Error(`Failed to fetch Fontshare fonts: ${String(error)}`);
-    }
-}
-
-/**
- * Fetch font by slug
- * Convenience function for fetching a single font
- *
- * @param slug - Font slug (e.g., "satoshi", "general-sans")
- * @returns Promise resolving to Fontshare font item
- *
- * @example
- * ```ts
- * const satoshi = await fetchFontshareFontBySlug('satoshi');
- * ```
- */
-export async function fetchFontshareFontBySlug(
-    slug: string,
-): Promise<FontshareFont | undefined> {
-    const response = await fetchFontshareFonts();
-    return response.fonts.find(font => font.slug === slug);
-}
-
-/**
- * Fetch all fonts from Fontshare
- * Convenience function for fetching all available fonts
- * Uses pagination to get all items
- *
- * @returns Promise resolving to all Fontshare fonts
- *
- * @example
- * ```ts
- * const allFonts = await fetchAllFontshareFonts();
- * console.log(`Found ${allFonts.fonts.length} fonts`);
- * ```
- */
-export async function fetchAllFontshareFonts(
-    params: FontshareParams = {},
-): Promise<FontshareResponse> {
-    const allFonts: FontshareFont[] = [];
-    let page = 1;
-    const limit = 100; // Max items per page
-
-    while (true) {
-        const response = await fetchFontshareFonts({
-            ...params,
-            page,
-            limit,
-        });
-
-        allFonts.push(...response.fonts);
-
-        // Check if we've fetched all items
-        if (response.fonts.length < limit) {
-            break;
-        }
-
-        page++;
-    }
-
-    // Return first response with all items combined
-    const firstResponse = await fetchFontshareFonts({ ...params, page: 1, limit });
-
-    return {
-        ...firstResponse,
-        fonts: allFonts,
-    };
-}

src/entities/Font/api/google/googleFonts.ts

@@ -1,127 +0,0 @@
-/**
- * Google Fonts API client
- *
- * Handles API requests to Google Fonts API for fetching font metadata.
- * Provides error handling, retry logic, and type-safe responses.
- *
- * Pagination: The Google Fonts API does NOT support pagination parameters.
- * All fonts matching the query are returned in a single response.
- * Use category, subset, or sort filters to reduce the result set if needed.
- *
- * @see https://developers.google.com/fonts/docs/developer_api
- */
-
-import { api } from '$shared/api/api';
-import { buildQueryString } from '$shared/lib/utils';
-import type { QueryParams } from '$shared/lib/utils';
-import type {
-    FontItem,
-    GoogleFontsApiModel,
-} from '../../model/types/google';
-
-/**
- * Google Fonts API parameters
- */
-export interface GoogleFontsParams extends QueryParams {
-    /**
-     * Google Fonts API key (required for Google Fonts API v1)
-     */
-    key?: string;
-    /**
-     * Font family name (to fetch specific font)
-     */
-    family?: string;
-    /**
-     * Font category filter (e.g., "sans-serif", "serif", "display")
-     */
-    category?: string;
-    /**
-     * Character subset filter (e.g., "latin", "latin-ext", "cyrillic")
-     */
-    subset?: string;
-    /**
-     * Sort order for results
-     */
-    sort?: 'alpha' | 'date' | 'popularity' | 'style' | 'trending';
-    /**
-     * Cap the number of fonts returned
-     */
-    capability?: 'VF' | 'WOFF2';
-}
-
-/**
- * Google Fonts API response wrapper
- * Re-exported from model/types/google for backward compatibility
- */
-export type GoogleFontsResponse = GoogleFontsApiModel;
-
-/**
- * Simplified font item from Google Fonts API
- * Re-exported from model/types/google for backward compatibility
- */
-export type GoogleFontItem = FontItem;
-
-/**
- * Google Fonts API base URL
- * Note: Google Fonts API v1 requires an API key. For development/testing without a key,
- * fonts may not load properly.
- */
-const GOOGLE_FONTS_API_URL = 'https://www.googleapis.com/webfonts/v1/webfonts' as const;
-
-/**
- * Fetch fonts from Google Fonts API
- *
- * @param params - Query parameters for filtering fonts
- * @returns Promise resolving to Google Fonts API response
- * @throws ApiError when request fails
- *
- * @example
- * ```ts
- * // Fetch all sans-serif fonts sorted by popularity
- * const response = await fetchGoogleFonts({
- *     category: 'sans-serif',
- *     sort: 'popularity'
- * });
- *
- * // Fetch specific font family
- * const robotoResponse = await fetchGoogleFonts({
- *     family: 'Roboto'
- * });
- * ```
- */
-export async function fetchGoogleFonts(
-    params: GoogleFontsParams = {},
-): Promise<GoogleFontsResponse> {
-    const queryString = buildQueryString(params);
-    const url = `${GOOGLE_FONTS_API_URL}${queryString}`;
-
-    try {
-        const response = await api.get<GoogleFontsResponse>(url);
-        return response.data;
-    } catch (error) {
-        // Re-throw ApiError with context
-        if (error instanceof Error) {
-            throw error;
-        }
-        throw new Error(`Failed to fetch Google Fonts: ${String(error)}`);
-    }
-}
-
-/**
- * Fetch font by family name
- * Convenience function for fetching a single font
- *
- * @param family - Font family name (e.g., "Roboto")
- * @returns Promise resolving to Google Font item
- *
- * @example
- * ```ts
- * const roboto = await fetchGoogleFontFamily('Roboto');
- * ```
- */
-export async function fetchGoogleFontFamily(
-    family: string,
-): Promise<GoogleFontItem | undefined> {
-    const response = await fetchGoogleFonts({ family });
-    return response.items.find(item => item.family === family);
-}

src/entities/Font/api/index.ts

@@ -4,35 +4,14 @@
  * Exports API clients and normalization utilities
  */
 
-// Proxy API (PRIMARY - NEW)
+// Proxy API (primary)
 export {
     fetchFontsByIds,
     fetchProxyFontById,
     fetchProxyFonts,
+    seedFontCache,
 } from './proxy/proxyFonts';
 export type {
     ProxyFontsParams,
     ProxyFontsResponse,
 } from './proxy/proxyFonts';
-
-// Google Fonts API (DEPRECATED - kept for backward compatibility)
-export {
-    fetchGoogleFontFamily,
-    fetchGoogleFonts,
-} from './google/googleFonts';
-export type {
-    GoogleFontItem,
-    GoogleFontsParams,
-    GoogleFontsResponse,
-} from './google/googleFonts';
-
-// Fontshare API (DEPRECATED - kept for backward compatibility)
-export {
-    fetchAllFontshareFonts,
-    fetchFontshareFontBySlug,
-    fetchFontshareFonts,
-} from './fontshare/fontshare';
-export type {
-    FontshareParams,
-    FontshareResponse,
-} from './fontshare/fontshare';

src/entities/Font/api/proxy/proxyFonts.test.ts

@@ -0,0 +1,211 @@
+/**
+ * Tests for proxy API client
+ */
+
+import {
+    beforeEach,
+    describe,
+    expect,
+    test,
+    vi,
+} from 'vitest';
+import type { UnifiedFont } from '../../model/types';
+import type { ProxyFontsResponse } from './proxyFonts';
+
+vi.mock('$shared/api/api', () => ({
+    api: {
+        get: vi.fn(),
+    },
+}));
+
+import { api } from '$shared/api/api';
+import { getQueryClient } from '$shared/api/queryClient';
+
+const queryClient = getQueryClient();
+import { fontKeys } from '$shared/api/queryKeys';
+import { FontResponseError } from '../../lib/errors/errors';
+import {
+    fetchFontsByIds,
+    fetchProxyFontById,
+    fetchProxyFonts,
+    seedFontCache,
+} from './proxyFonts';
+
+const PROXY_API_URL = 'https://api.glyphdiff.com/api/v1/fonts';
+
+function createMockFont(overrides: Partial<UnifiedFont> = {}): UnifiedFont {
+    return {
+        id: 'roboto',
+        family: 'Roboto',
+        provider: 'google',
+        category: 'sans-serif',
+        variants: [],
+        subsets: [],
+        ...overrides,
+    } as UnifiedFont;
+}
+
+function mockApiGet<T>(data: T) {
+    vi.mocked(api.get).mockResolvedValueOnce({ data, status: 200 });
+}
+
+describe('proxyFonts', () => {
+    beforeEach(() => {
+        vi.mocked(api.get).mockReset();
+        queryClient.clear();
+    });
+
+    describe('fetchProxyFonts', () => {
+        test('should fetch fonts with no params', async () => {
+            const mockResponse: ProxyFontsResponse = {
+                fonts: [createMockFont()],
+                total: 1,
+                limit: 50,
+                offset: 0,
+            };
+            mockApiGet(mockResponse);
+
+            const result = await fetchProxyFonts();
+
+            expect(api.get).toHaveBeenCalledWith(PROXY_API_URL);
+            expect(result).toEqual(mockResponse);
+        });
+
+        test('should build URL with query params', async () => {
+            const mockResponse: ProxyFontsResponse = {
+                fonts: [createMockFont()],
+                total: 1,
+                limit: 20,
+                offset: 0,
+            };
+            mockApiGet(mockResponse);
+
+            await fetchProxyFonts({ provider: 'google', category: 'sans-serif', limit: 20, offset: 0 });
+
+            const calledUrl = vi.mocked(api.get).mock.calls[0][0];
+            expect(calledUrl).toContain('provider=google');
+            expect(calledUrl).toContain('category=sans-serif');
+            expect(calledUrl).toContain('limit=20');
+            expect(calledUrl).toContain('offset=0');
+        });
+
+        test('should throw FontResponseError on invalid response (missing fonts array)', async () => {
+            mockApiGet({ total: 0 });
+
+            await expect(fetchProxyFonts()).rejects.toSatisfy(
+                e => e instanceof FontResponseError && e.field === 'response.fonts',
+            );
+        });
+
+        test('should throw FontResponseError on null response data', async () => {
+            vi.mocked(api.get).mockResolvedValueOnce({ data: null, status: 200 });
+
+            await expect(fetchProxyFonts()).rejects.toSatisfy(
+                e => e instanceof FontResponseError && e.field === 'response',
+            );
+        });
+    });
+
+    describe('fetchProxyFontById', () => {
+        test('should return font matching the ID', async () => {
+            const targetFont = createMockFont({ id: 'satoshi', name: 'Satoshi' });
+            const mockResponse: ProxyFontsResponse = {
+                fonts: [createMockFont(), targetFont],
+                total: 2,
+                limit: 1000,
+                offset: 0,
+            };
+            mockApiGet(mockResponse);
+
+            const result = await fetchProxyFontById('satoshi');
+
+            expect(result).toEqual(targetFont);
+        });
+
+        test('should return undefined when font not found', async () => {
+            const mockResponse: ProxyFontsResponse = {
+                fonts: [createMockFont()],
+                total: 1,
+                limit: 1000,
+                offset: 0,
+            };
+            mockApiGet(mockResponse);
+
+            const result = await fetchProxyFontById('nonexistent');
+
+            expect(result).toBeUndefined();
+        });
+
+        test('should search with the ID as query param', async () => {
+            const mockResponse: ProxyFontsResponse = {
+                fonts: [],
+                total: 0,
+                limit: 1000,
+                offset: 0,
+            };
+            mockApiGet(mockResponse);
+
+            await fetchProxyFontById('Roboto');
+
+            const calledUrl = vi.mocked(api.get).mock.calls[0][0];
+            expect(calledUrl).toContain('limit=1000');
+            expect(calledUrl).toContain('q=Roboto');
+        });
+    });
+
+    describe('fetchFontsByIds', () => {
+        test('should return empty array for empty input', async () => {
+            const result = await fetchFontsByIds([]);
+
+            expect(result).toEqual([]);
+            expect(api.get).not.toHaveBeenCalled();
+        });
+
+        test('should call batch endpoint with comma-separated IDs', async () => {
+            const fonts = [createMockFont({ id: 'roboto' }), createMockFont({ id: 'satoshi' })];
+            mockApiGet(fonts);
+
+            const result = await fetchFontsByIds(['roboto', 'satoshi']);
+
+            expect(api.get).toHaveBeenCalledWith(`${PROXY_API_URL}/batch?ids=roboto,satoshi`);
+            expect(result).toEqual(fonts);
+        });
+
+        test('should return empty array when response data is nullish', async () => {
+            vi.mocked(api.get).mockResolvedValueOnce({ data: null, status: 200 });
+
+            const result = await fetchFontsByIds(['roboto']);
+
+            expect(result).toEqual([]);
+        });
+    });
+
+    describe('seedFontCache', () => {
+        test('should populate cache with multiple fonts', () => {
+            const fonts = [
+                createMockFont({ id: '1', name: 'A' }),
+                createMockFont({ id: '2', name: 'B' }),
+            ];
+            seedFontCache(fonts);
+            expect(queryClient.getQueryData(fontKeys.detail('1'))).toEqual(fonts[0]);
+            expect(queryClient.getQueryData(fontKeys.detail('2'))).toEqual(fonts[1]);
+        });
+
+        test('should update existing cached fonts with new data', () => {
+            const id = 'update-me';
+            queryClient.setQueryData(fontKeys.detail(id), createMockFont({ id, name: 'Old' }));
+
+            const updated = createMockFont({ id, name: 'New' });
+            seedFontCache([updated]);
+
+            expect(queryClient.getQueryData(fontKeys.detail(id))).toEqual(updated);
+        });
+
+        test('should handle empty input arrays gracefully', () => {
+            const spy = vi.spyOn(queryClient, 'setQueryData');
+            seedFontCache([]);
+            expect(spy).not.toHaveBeenCalled();
+            spy.mockRestore();
+        });
+    });
+});

src/entities/Font/api/proxy/proxyFonts.ts

@@ -7,59 +7,67 @@
  * Proxy API normalizes font data from Google Fonts and Fontshare into a single
  * unified format, eliminating the need for client-side normalization.
  *
- * Fallback: If proxy API fails, falls back to Fontshare API for development.
- *
  * @see https://api.glyphdiff.com/api/v1/fonts
  */
 
 import { api } from '$shared/api/api';
+import { getQueryClient } from '$shared/api/queryClient';
+import { fontKeys } from '$shared/api/queryKeys';
 import { buildQueryString } from '$shared/lib/utils';
 import type { QueryParams } from '$shared/lib/utils';
+import { FontResponseError } from '../../lib/errors/errors';
 import type { UnifiedFont } from '../../model/types';
-import type {
-    FontCategory,
-    FontSubset,
-} from '../../model/types';
 
 /**
- * Proxy API base URL
+ * Normalizes cache by seeding individual font entries from collection responses.
+ * This ensures that a font fetched in a list or batch is available via its detail key.
+ *
+ * @param fonts - Array of fonts to cache
  */
-const PROXY_API_URL = 'https://api.glyphdiff.com/api/v1/fonts' as const;
+export function seedFontCache(fonts: UnifiedFont[]): void {
+    fonts.forEach(font => {
+        getQueryClient().setQueryData(fontKeys.detail(font.id), font);
+    });
+}
+
+import { API_ENDPOINTS } from '$shared/api/endpoints';
 
 /**
- * Whether to use proxy API (true) or fallback (false)
- *
- * Set to true when your proxy API is ready:
- *   const USE_PROXY_API = true;
- *
- * Set to false to use Fontshare API as fallback during development:
- *   const USE_PROXY_API = false;
- *
- * The app will automatically fall back to Fontshare API if the proxy fails.
+ * Proxy API endpoint for font resources.
  */
-const USE_PROXY_API = true;
+const PROXY_API_URL = API_ENDPOINTS.fonts;
 
 /**
  * Proxy API parameters
  *
  * Maps directly to the proxy API query parameters
+ *
+ * UPDATED: Now supports array values for filters
  */
 export interface ProxyFontsParams extends QueryParams {
     /**
-     * Font provider filter ("google" or "fontshare")
-     * Omit to fetch from both providers
+     * Font provider filter
+     *
+     * NEW: Supports array of providers (e.g., ["google", "fontshare"])
+     * Backward compatible: Single value still works
      */
-    provider?: 'google' | 'fontshare';
+    providers?: string[] | string;
 
     /**
      * Font category filter
+     *
+     * NEW: Supports array of categories (e.g., ["serif", "sans-serif"])
+     * Backward compatible: Single value still works
      */
-    category?: FontCategory;
+    categories?: string[] | string;
 
     /**
      * Character subset filter
+     *
+     * NEW: Supports array of subsets (e.g., ["latin", "cyrillic"])
+     * Backward compatible: Single value still works
      */
-    subset?: FontSubset;
+    subsets?: string[] | string;
 
     /**
      * Search query (e.g., "roboto", "satoshi")
@@ -89,27 +97,38 @@ export interface ProxyFontsParams extends QueryParams {
 /**
  * Proxy API response
  *
- * Includes pagination metadata alongside font data
+ * Includes pagination metadata alongside font data.
+ *
+ * Contract: `fonts` is always an array — never `null` or omitted, even when
+ * `total === 0`. Returning `null` on the wire is a backend regression and
+ * surfaces as FontResponseError (non-retryable) on the client.
  */
 export interface ProxyFontsResponse {
-    /** Array of unified font objects */
+    /**
+     * List of font objects returned by the proxy.
+     * Always an array; empty when no matches.
+     */
     fonts: UnifiedFont[];
 
-    /** Total number of fonts matching the query */
+    /**
+     * Total number of matching fonts (ignoring limit/offset)
+     */
     total: number;
 
-    /** Limit used for this request */
+    /**
+     * Page size used for the request
+     */
     limit: number;
 
-    /** Offset used for this request */
+    /**
+     * Start index for the result set
+     */
     offset: number;
 }
 
 /**
  * Fetch fonts from proxy API
  *
- * If proxy API fails or is unavailable, falls back to Fontshare API for development.
- *
  * @param params - Query parameters for filtering and pagination
  * @returns Promise resolving to proxy API response
  * @throws ApiError when request fails
@@ -138,84 +157,19 @@ export interface ProxyFontsResponse {
 export async function fetchProxyFonts(
     params: ProxyFontsParams = {},
 ): Promise<ProxyFontsResponse> {
-    // Try proxy API first if enabled
-    if (USE_PROXY_API) {
-        try {
     const queryString = buildQueryString(params);
     const url = `${PROXY_API_URL}${queryString}`;
 
-            console.log('[fetchProxyFonts] Fetching from proxy API', { params, url });
-
     const response = await api.get<ProxyFontsResponse>(url);
 
-            // Validate response has fonts array
-            if (!response.data || !Array.isArray(response.data.fonts)) {
-                console.error('[fetchProxyFonts] Invalid response from proxy API', response.data);
-                throw new Error('Proxy API returned invalid response');
+    if (!response.data) {
+        throw new FontResponseError('response', response.data);
+    }
+    if (!Array.isArray(response.data.fonts)) {
+        throw new FontResponseError('response.fonts', response.data.fonts);
     }
 
-            console.log('[fetchProxyFonts] Proxy API success', {
-                count: response.data.fonts.length,
-            });
     return response.data;
-        } catch (error) {
-            console.warn('[fetchProxyFonts] Proxy API failed, using fallback', error);
-
-            // Check if it's a network error or proxy not available
-            const isNetworkError = error instanceof Error
-                && (error.message.includes('Failed to fetch')
-                    || error.message.includes('Network')
-                    || error.message.includes('404')
-                    || error.message.includes('500'));
-
-            if (isNetworkError) {
-                // Fall back to Fontshare API
-                console.log('[fetchProxyFonts] Using Fontshare API as fallback');
-                return await fetchFontshareFallback(params);
-            }
-
-            // Re-throw other errors
-            if (error instanceof Error) {
-                throw error;
-            }
-            throw new Error(`Failed to fetch fonts from proxy API: ${String(error)}`);
-        }
-    }
-
-    // Use Fontshare API directly
-    console.log('[fetchProxyFonts] Using Fontshare API (proxy disabled)');
-    return await fetchFontshareFallback(params);
-}
-
-/**
- * Fallback to Fontshare API when proxy is unavailable
- *
- * Maps proxy API params to Fontshare API params and normalizes response
- */
-async function fetchFontshareFallback(
-    params: ProxyFontsParams,
-): Promise<ProxyFontsResponse> {
-    // Import dynamically to avoid circular dependency
-    const { fetchFontshareFonts } = await import('$entities/Font/api/fontshare/fontshare');
-    const { normalizeFontshareFonts } = await import('$entities/Font/lib/normalize/normalize');
-
-    // Map proxy params to Fontshare params
-    const fontshareParams = {
-        q: params.q,
-        categories: params.category ? [params.category] : undefined,
-        page: params.offset ? Math.floor(params.offset / (params.limit || 50)) + 1 : undefined,
-        limit: params.limit,
-    };
-
-    const response = await fetchFontshareFonts(fontshareParams);
-    const normalizedFonts = normalizeFontshareFonts(response.fonts);
-
-    return {
-        fonts: normalizedFonts,
-        total: response.count_total,
-        limit: params.limit || response.count,
-        offset: params.offset || 0,
-    };
 }
 
 /**
@@ -254,26 +208,13 @@ export async function fetchProxyFontById(
  * @returns Promise resolving to an array of fonts
  */
 export async function fetchFontsByIds(ids: string[]): Promise<UnifiedFont[]> {
-    if (ids.length === 0) return [];
+    if (ids.length === 0) {
+        return [];
+    }
 
-    // Use proxy API if enabled
-    if (USE_PROXY_API) {
     const queryString = ids.join(',');
     const url = `${PROXY_API_URL}/batch?ids=${queryString}`;
 
-        try {
     const response = await api.get<UnifiedFont[]>(url);
     return response.data ?? [];
-        } catch (error) {
-            console.warn('[fetchFontsByIds] Proxy API batch fetch failed, falling back', error);
-            // Fallthrough to fallback
-        }
-    }
-
-    // Fallback: Fetch individually (not efficient but functional for fallback)
-    const results = await Promise.all(
-        ids.map(id => fetchProxyFontById(id)),
-    );
-
-    return results.filter((f): f is UnifiedFont => !!f);
 }

src/entities/Font/domain/DualFontLayout/DualFontLayout.test.ts

@@ -0,0 +1,95 @@
+// @vitest-environment jsdom
+import { installCanvasMock } from '$shared/lib/helpers/__mocks__/canvas';
+import { clearCache } from '@chenglou/pretext';
+import {
+    beforeEach,
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { DualFontLayout } from './DualFontLayout';
+
+// FontA: 10px per character. FontB: 15px per character.
+// The mock dispatches on whether the font string contains 'FontA' or 'FontB'.
+const FONT_A_WIDTH = 10;
+const FONT_B_WIDTH = 15;
+
+function fontWidthFactory(font: string, text: string): number {
+    const perChar = font.includes('FontA') ? FONT_A_WIDTH : FONT_B_WIDTH;
+    return text.length * perChar;
+}
+
+describe('DualFontLayout', () => {
+    let layout: DualFontLayout;
+
+    beforeEach(() => {
+        installCanvasMock(fontWidthFactory);
+        clearCache();
+        layout = new DualFontLayout();
+    });
+
+    it('returns empty result for empty string', () => {
+        const result = layout.layout('', '400 16px "FontA"', '400 16px "FontB"', 500, 20);
+        expect(result.lines).toHaveLength(0);
+        expect(result.totalHeight).toBe(0);
+    });
+
+    it('uses worst-case width across both fonts to determine line breaks', () => {
+        // 'AB CD' — two 2-char words separated by a space.
+        // FontA: 'AB'=20px, 'CD'=20px. Both fit in 25px? No: 'AB CD' = 50px total.
+        // FontB: 'AB'=30px, 'CD'=30px. Width 35px forces wrap after 'AB '.
+        // Unified must use FontB widths — so it must wrap at the same place FontB wraps.
+        const result = layout.layout('AB CD', '400 16px "FontA"', '400 16px "FontB"', 35, 20);
+        expect(result.lines.length).toBeGreaterThan(1);
+        // First line text must not include both words.
+        expect(result.lines[0].text).not.toContain('CD');
+    });
+
+    it('provides xA and xB offsets for both fonts on a single line', () => {
+        // 'ABC' fits in 500px for both fonts.
+        // FontA: A@0(w=10), B@10(w=10), C@20(w=10)
+        // FontB: A@0(w=15), B@15(w=15), C@30(w=15)
+        const result = layout.layout('ABC', '400 16px "FontA"', '400 16px "FontB"', 500, 20);
+        const chars = result.lines[0].chars;
+
+        expect(chars).toHaveLength(3);
+
+        expect(chars[0].xA).toBe(0);
+        expect(chars[0].widthA).toBe(FONT_A_WIDTH);
+        expect(chars[0].xB).toBe(0);
+        expect(chars[0].widthB).toBe(FONT_B_WIDTH);
+
+        expect(chars[1].xA).toBe(FONT_A_WIDTH); // 10
+        expect(chars[1].widthA).toBe(FONT_A_WIDTH);
+        expect(chars[1].xB).toBe(FONT_B_WIDTH); // 15
+        expect(chars[1].widthB).toBe(FONT_B_WIDTH);
+
+        expect(chars[2].xA).toBe(FONT_A_WIDTH * 2); // 20
+        expect(chars[2].xB).toBe(FONT_B_WIDTH * 2); // 30
+    });
+
+    it('returns cached result when called again with same arguments', () => {
+        const r1 = layout.layout('ABC', '400 16px "FontA"', '400 16px "FontB"', 500, 20);
+        const r2 = layout.layout('ABC', '400 16px "FontA"', '400 16px "FontB"', 500, 20);
+        expect(r2).toBe(r1); // strict reference equality — same object
+    });
+
+    it('re-computes when text changes', () => {
+        const r1 = layout.layout('ABC', '400 16px "FontA"', '400 16px "FontB"', 500, 20);
+        const r2 = layout.layout('DEF', '400 16px "FontA"', '400 16px "FontB"', 500, 20);
+        expect(r2).not.toBe(r1);
+        expect(r2.lines[0].text).not.toBe(r1.lines[0].text);
+    });
+
+    it('re-computes when width changes', () => {
+        const r1 = layout.layout('Hello World', '400 16px "FontA"', '400 16px "FontB"', 500, 20);
+        const r2 = layout.layout('Hello World', '400 16px "FontA"', '400 16px "FontB"', 60, 20);
+        expect(r2).not.toBe(r1);
+    });
+
+    it('re-computes when fontA changes', () => {
+        const r1 = layout.layout('ABC', '400 16px "FontA"', '400 16px "FontB"', 500, 20);
+        const r2 = layout.layout('ABC', '400 24px "FontA"', '400 16px "FontB"', 500, 20);
+        expect(r2).not.toBe(r1);
+    });
+});

src/entities/Font/domain/DualFontLayout/DualFontLayout.ts

@@ -0,0 +1,278 @@
+import {
+    type PreparedTextWithSegments,
+    layoutWithLines,
+    prepareWithSegments,
+} from '@chenglou/pretext';
+
+/**
+ * Default render size in px when callers omit the `size` arg on `layout()`.
+ */
+const DEFAULT_RENDER_SIZE_PX = 16;
+
+/**
+ * Per-grapheme data computed during dual-font layout. Internal to the engine;
+ * consumed by computeLineRenderModel to derive the per-frame render model.
+ */
+export interface ComparisonChar {
+    /**
+     * Grapheme cluster (may be >1 code unit for emoji, combining marks).
+     */
+    char: string;
+    /**
+     * X offset from line start in fontA, pixels.
+     */
+    xA: number;
+    /**
+     * Advance width of this grapheme in fontA, pixels.
+     */
+    widthA: number;
+    /**
+     * X offset from line start in fontB, pixels.
+     */
+    xB: number;
+    /**
+     * Advance width of this grapheme in fontB, pixels.
+     */
+    widthB: number;
+}
+
+/**
+ * A single laid-out line. `chars` carries the per-grapheme data needed by
+ * computeLineRenderModel. Consumers should not iterate it directly.
+ */
+export interface ComparisonLine {
+    /**
+     * Full text of this line as returned by pretext.
+     */
+    text: string;
+    /**
+     * Rendered width in pixels — maximum across fontA and fontB.
+     */
+    width: number;
+    /**
+     * Per-grapheme metadata for both fonts.
+     */
+    chars: ComparisonChar[];
+}
+
+/**
+ * Aggregated output of a dual-font layout pass.
+ */
+export interface ComparisonResult {
+    /**
+     * Per-line grapheme data. Empty when input text is empty.
+     */
+    lines: ComparisonLine[];
+    /**
+     * Total height in pixels.
+     */
+    totalHeight: number;
+}
+
+/**
+ * Dual-font text layout engine backed by `@chenglou/pretext`.
+ *
+ * Computes identical line breaks for two fonts simultaneously by constructing a
+ * "unified" prepared-text object whose per-glyph widths are the worst-case maximum
+ * of font A and font B. This guarantees that both fonts wrap at exactly the same
+ * positions, making side-by-side or slider comparison visually coherent.
+ *
+ * Relies on pretext's published structural fields on `PreparedTextWithSegments`
+ * (`widths`, `breakableFitAdvances`, `lineEndFitAdvances`, `lineEndPaintAdvances`)
+ * which are exposed via the `PreparedCore` intersection in `@chenglou/pretext@0.0.6`.
+ *
+ * **Two-level caching strategy**
+ * 1. Font-change cache (`#preparedA`, `#preparedB`, `#unifiedPrepared`): rebuilt only
+ *    when `text`, `fontA`, or `fontB` changes. `prepareWithSegments` is expensive
+ *    (canvas measurement), so this avoids re-measuring during slider interaction.
+ * 2. Layout cache (`#lastResult`): rebuilt when `width` or `lineHeight` changes but
+ *    the fonts have not changed. Line-breaking is cheap relative to measurement, but
+ *    still worth skipping on every render tick.
+ *
+ * Per-frame slider state derivation lives in `computeLineRenderModel`, not on the
+ * class. This class is pure layout + caching; it holds no reactive state.
+ */
+export class DualFontLayout {
+    #segmenter: Intl.Segmenter;
+
+    // Cached prepared data
+    #preparedA: PreparedTextWithSegments | null = null;
+    #preparedB: PreparedTextWithSegments | null = null;
+    #unifiedPrepared: PreparedTextWithSegments | null = null;
+
+    #lastText = '';
+    #lastFontA = '';
+    #lastFontB = '';
+    #lastSpacing = 0;
+    #lastSize = 0;
+
+    // Cached layout results
+    #lastWidth = -1;
+    #lastLineHeight = -1;
+    #lastResult: ComparisonResult | null = null;
+
+    constructor(locale?: string) {
+        this.#segmenter = new Intl.Segmenter(locale, { granularity: 'grapheme' });
+    }
+
+    /**
+     * Lay out `text` using both fonts within `width` pixels.
+     *
+     * Line breaks are determined by the worst-case (maximum) glyph widths across
+     * both fonts, so both fonts always wrap at identical positions.
+     *
+     * @param text       Raw text to lay out.
+     * @param fontA      CSS font string for the first font: `"weight sizepx \"family\""`.
+     * @param fontB      CSS font string for the second font: `"weight sizepx \"family\""`.
+     * @param width      Available line width in pixels.
+     * @param lineHeight Line height in pixels (passed directly to pretext).
+     * @param spacing    Letter spacing in em (from typography settings).
+     * @param size       Current font size in pixels (used to convert spacing em to px).
+     * @returns          Per-line grapheme data for both fonts. Empty `lines` when `text` is empty.
+     */
+    layout(
+        text: string,
+        fontA: string,
+        fontB: string,
+        width: number,
+        lineHeight: number,
+        spacing: number = 0,
+        size: number = DEFAULT_RENDER_SIZE_PX,
+    ): ComparisonResult {
+        if (!text) {
+            return { lines: [], totalHeight: 0 };
+        }
+
+        const spacingPx = spacing * size;
+
+        const isFontChange = text !== this.#lastText
+            || fontA !== this.#lastFontA
+            || fontB !== this.#lastFontB
+            || spacing !== this.#lastSpacing
+            || size !== this.#lastSize;
+
+        const isLayoutChange = width !== this.#lastWidth || lineHeight !== this.#lastLineHeight;
+
+        if (!isFontChange && !isLayoutChange && this.#lastResult) {
+            return this.#lastResult;
+        }
+
+        // 1. Prepare (or use cache)
+        if (isFontChange) {
+            this.#preparedA = prepareWithSegments(text, fontA);
+            this.#preparedB = prepareWithSegments(text, fontB);
+            this.#unifiedPrepared = this.#createUnifiedPrepared(this.#preparedA, this.#preparedB, spacingPx);
+
+            this.#lastText = text;
+            this.#lastFontA = fontA;
+            this.#lastFontB = fontB;
+            this.#lastSpacing = spacing;
+            this.#lastSize = size;
+        }
+
+        if (!this.#unifiedPrepared || !this.#preparedA || !this.#preparedB) {
+            return { lines: [], totalHeight: 0 };
+        }
+
+        const { lines, height } = layoutWithLines(this.#unifiedPrepared, width, lineHeight);
+
+        // 3. Map results back to both fonts
+        const preparedA = this.#preparedA;
+        const preparedB = this.#preparedB;
+        const resultLines: ComparisonLine[] = lines.map(line => {
+            const chars: ComparisonChar[] = [];
+            let currentXA = 0;
+            let currentXB = 0;
+
+            const start = line.start;
+            const end = line.end;
+
+            for (let sIdx = start.segmentIndex; sIdx <= end.segmentIndex; sIdx++) {
+                const segmentText = preparedA.segments[sIdx];
+                if (segmentText === undefined) {
+                    continue;
+                }
+
+                const graphemes = Array.from(this.#segmenter.segment(segmentText), s => s.segment);
+
+                const advA = preparedA.breakableFitAdvances[sIdx];
+                const advB = preparedB.breakableFitAdvances[sIdx];
+
+                const gStart = sIdx === start.segmentIndex ? start.graphemeIndex : 0;
+                const gEnd = sIdx === end.segmentIndex ? end.graphemeIndex : graphemes.length;
+
+                for (let gIdx = gStart; gIdx < gEnd; gIdx++) {
+                    const char = graphemes[gIdx];
+                    let wA = advA != null ? advA[gIdx]! : preparedA.widths[sIdx]!;
+                    let wB = advB != null ? advB[gIdx]! : preparedB.widths[sIdx]!;
+
+                    // Apply letter spacing (tracking) to the width of each character
+                    wA += spacingPx;
+                    wB += spacingPx;
+
+                    chars.push({
+                        char,
+                        xA: currentXA,
+                        widthA: wA,
+                        xB: currentXB,
+                        widthB: wB,
+                    });
+                    currentXA += wA;
+                    currentXB += wB;
+                }
+            }
+
+            return {
+                text: line.text,
+                width: line.width,
+                chars,
+            };
+        });
+
+        this.#lastWidth = width;
+        this.#lastLineHeight = lineHeight;
+        this.#lastResult = {
+            lines: resultLines,
+            totalHeight: height,
+        };
+
+        return this.#lastResult;
+    }
+
+    /**
+     * Merge two prepared texts into a worst-case unified version so both fonts
+     * wrap at identical positions. Per-segment widths are the elementwise max
+     * across both fonts, with `spacingPx` added to model letter-spacing.
+     */
+    #createUnifiedPrepared(
+        a: PreparedTextWithSegments,
+        b: PreparedTextWithSegments,
+        spacingPx: number = 0,
+    ): PreparedTextWithSegments {
+        const unified: PreparedTextWithSegments = { ...a };
+
+        unified.widths = a.widths.map((w, i) => Math.max(w, b.widths[i]) + spacingPx);
+        unified.lineEndFitAdvances = a.lineEndFitAdvances.map((w, i) =>
+            Math.max(w, b.lineEndFitAdvances[i]) + spacingPx
+        );
+        unified.lineEndPaintAdvances = a.lineEndPaintAdvances.map((w, i) =>
+            Math.max(w, b.lineEndPaintAdvances[i]) + spacingPx
+        );
+
+        unified.breakableFitAdvances = a.breakableFitAdvances.map((advA, i) => {
+            const advB = b.breakableFitAdvances[i];
+            if (!advA && !advB) {
+                return null;
+            }
+            if (!advA) {
+                return advB!.map(w => w + spacingPx);
+            }
+            if (!advB) {
+                return advA.map(w => w + spacingPx);
+            }
+            return advA.map((w, j) => Math.max(w, advB[j]) + spacingPx);
+        });
+
+        return unified;
+    }
+}

src/entities/Font/domain/computeLineRenderModel/computeLineRenderModel.test.ts

@@ -0,0 +1,220 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import type { ComparisonLine } from '../DualFontLayout/DualFontLayout';
+import {
+    type LineRenderModel,
+    computeLineRenderModel,
+    findSplitIndex,
+} from './computeLineRenderModel';
+
+/**
+ * Build a ComparisonLine fixture with given per-char widths. xA/xB are
+ * cumulative prefix sums of widthA/widthB respectively.
+ */
+function makeLine(
+    chars: { char: string; widthA: number; widthB: number }[],
+): ComparisonLine {
+    let xA = 0;
+    let xB = 0;
+    const out: ComparisonLine = {
+        text: chars.map(c => c.char).join(''),
+        width: chars.reduce((s, c) => s + Math.max(c.widthA, c.widthB), 0),
+        chars: chars.map(c => {
+            const entry = {
+                char: c.char,
+                xA,
+                xB,
+                widthA: c.widthA,
+                widthB: c.widthB,
+            };
+            xA += c.widthA;
+            xB += c.widthB;
+            return entry;
+        }),
+    };
+    return out;
+}
+
+/**
+ * Test helper: compute split + render model in one step, matching the
+ * SliderArea call site shape.
+ */
+function compute(
+    line: ComparisonLine,
+    sliderPos: number,
+    containerWidth: number,
+    windowSize: number,
+): LineRenderModel {
+    const split = findSplitIndex(line, sliderPos, containerWidth);
+    return computeLineRenderModel(line, split, windowSize);
+}
+
+describe('computeLineRenderModel', () => {
+    it('returns empty model for an empty line', () => {
+        const line = makeLine([]);
+        const model = compute(line, 50, 500, 5);
+        expect(model.leftText).toBe('');
+        expect(model.windowChars).toEqual([]);
+        expect(model.rightText).toBe('');
+    });
+
+    it('places entire line in rightText when slider is at 0', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+        ]);
+        const model = compute(line, 0, 500, 0);
+        expect(model.leftText).toBe('');
+        expect(model.windowChars).toEqual([]);
+        expect(model.rightText).toBe('ABC');
+    });
+
+    it('places entire line in leftText when slider is at 100', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+        ]);
+        const model = compute(line, 100, 500, 0);
+        expect(model.leftText).toBe('ABC');
+        expect(model.windowChars).toEqual([]);
+        expect(model.rightText).toBe('');
+    });
+
+    it('splits line correctly with slider mid-line (window=0)', () => {
+        // Equal widths → line is centered. Container=300, total=30 → xOffset=135.
+        // Char thresholds (per the threshold formula in the design):
+        //   threshold[i] = xOffset + prefA[i] + widthA[i]/2
+        //   i=0: 135 + 0  + 5  = 140  → 140/300 = 46.67%
+        //   i=1: 135 + 10 + 5  = 150  → 150/300 = 50.00%
+        //   i=2: 135 + 20 + 5  = 160  → 160/300 = 53.33%
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+        ]);
+        // Slider just past B's threshold (50%) but not C's (53.33%).
+        const model = compute(line, 51, 300, 0);
+        expect(model.leftText).toBe('AB');
+        expect(model.rightText).toBe('C');
+    });
+
+    it('centers window of size 3 on the split index', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+            { char: 'D', widthA: 10, widthB: 10 },
+            { char: 'E', widthA: 10, widthB: 10 },
+        ]);
+        // Slider past A and B (~thresholds 43.33%, 46.67%); not past C (50%).
+        // split = 2 → halfWindow = 1 → windowStart = 1, windowEnd = 4
+        const model = compute(line, 48, 300, 3);
+        expect(model.leftText).toBe('A');
+        expect(model.windowChars.map(w => w.char)).toEqual(['B', 'C', 'D']);
+        expect(model.rightText).toBe('E');
+    });
+
+    it('clamps window at line start when slider is near 0', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+            { char: 'D', widthA: 10, widthB: 10 },
+            { char: 'E', widthA: 10, widthB: 10 },
+        ]);
+        const model = compute(line, 0, 300, 3);
+        expect(model.leftText).toBe('');
+        expect(model.windowChars.map(w => w.char)).toEqual(['A', 'B', 'C']);
+        expect(model.rightText).toBe('DE');
+    });
+
+    it('clamps window at line end when slider is near 100', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+            { char: 'D', widthA: 10, widthB: 10 },
+            { char: 'E', widthA: 10, widthB: 10 },
+        ]);
+        const model = compute(line, 100, 300, 3);
+        expect(model.leftText).toBe('AB');
+        expect(model.windowChars.map(w => w.char)).toEqual(['C', 'D', 'E']);
+        expect(model.rightText).toBe('');
+    });
+
+    it('treats whole line as window when line is shorter than windowSize', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+        ]);
+        const model = compute(line, 50, 300, 5);
+        expect(model.leftText).toBe('');
+        expect(model.windowChars.map(w => w.char)).toEqual(['A', 'B']);
+        expect(model.rightText).toBe('');
+    });
+
+    it('produces stable keys across slider movement within the same line', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+            { char: 'D', widthA: 10, widthB: 10 },
+            { char: 'E', widthA: 10, widthB: 10 },
+        ]);
+        const a = compute(line, 40, 300, 3);
+        const b = compute(line, 60, 300, 3);
+        // Chars that appear in both windows must carry identical keys.
+        for (const charA of a.windowChars) {
+            const charB = b.windowChars.find(w => w.char === charA.char);
+            if (charB !== undefined) {
+                expect(charB.key).toBe(charA.key);
+            }
+        }
+    });
+
+    it('marks isPast=true for chars before the split and false for chars after', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+            { char: 'D', widthA: 10, widthB: 10 },
+            { char: 'E', widthA: 10, widthB: 10 },
+        ]);
+        // split = 2 → A,B past; C,D,E not
+        const model = compute(line, 48, 300, 5);
+        const expected = new Map([['A', true], ['B', true], ['C', false], ['D', false], ['E', false]]);
+        for (const wc of model.windowChars) {
+            expect(wc.isPast).toBe(expected.get(wc.char));
+        }
+    });
+});
+
+describe('findSplitIndex', () => {
+    it('returns 0 for empty line', () => {
+        const line = makeLine([]);
+        expect(findSplitIndex(line, 50, 500)).toBe(0);
+    });
+
+    it('returns 0 when slider is before all char thresholds', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+        ]);
+        expect(findSplitIndex(line, 0, 300)).toBe(0);
+    });
+
+    it('returns chars.length when slider is past all char thresholds', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+        ]);
+        expect(findSplitIndex(line, 100, 300)).toBe(3);
+    });
+});

src/entities/Font/domain/computeLineRenderModel/computeLineRenderModel.ts

@@ -0,0 +1,133 @@
+import type { ComparisonLine } from '../DualFontLayout/DualFontLayout';
+
+/**
+ * Per-line render slice consumed by Line.svelte. The window is centered on the
+ * slider's split index and clamps at line boundaries.
+ */
+export interface LineRenderModel {
+    /**
+     * Chars before the window joined into a single string, rendered as one fontA text run.
+     */
+    leftText: string;
+    /**
+     * Window chars — each rendered as its own Character element with crossfade slots.
+     */
+    windowChars: Array<{
+        /**
+         * Stable key for Svelte keyed each — survives slider movement within the same line.
+         */
+        key: string;
+        /**
+         * Grapheme cluster to render.
+         */
+        char: string;
+        /**
+         * True once the slider has crossed this char's threshold.
+         */
+        isPast: boolean;
+    }>;
+    /**
+     * Chars after the window joined into a single string, rendered as one fontB text run.
+     */
+    rightText: string;
+}
+
+/**
+ * Returns the count of chars whose flip threshold the slider has crossed.
+ *
+ * Exposed as a separate step so consumers can pass the resulting primitive
+ * `split` across component boundaries: when split is unchanged tick-to-tick,
+ * downstream `$derived` reads of `computeLineRenderModel(line, split, ...)`
+ * short-circuit on value equality and skip re-rendering.
+ *
+ * For each candidate split `i`, the line's hypothetical width at that moment is
+ * `prefA[i] + widthA[i] + sufB[i+1]` (past chars in fontA, char `i` flipping, future
+ * chars in fontB). The threshold is the x of char `i`'s center in the centered line.
+ * Thresholds are monotonically non-decreasing in `i`, so the scan short-circuits on
+ * the first miss.
+ */
+export function findSplitIndex(
+    line: ComparisonLine,
+    sliderPos: number,
+    containerWidth: number,
+): number {
+    const chars = line.chars;
+    const n = chars.length;
+    if (n === 0) {
+        return 0;
+    }
+    const sliderX = (sliderPos / 100) * containerWidth;
+
+    const prefA = new Float64Array(n + 1);
+    const sufB = new Float64Array(n + 1);
+    for (let i = 0, j = n - 1; i < n; i++, j--) {
+        prefA[i + 1] = prefA[i] + chars[i].widthA;
+        sufB[j] = sufB[j + 1] + chars[j].widthB;
+    }
+
+    let split = 0;
+    for (let i = 0; i < n; i++) {
+        const totalWidth = prefA[i] + chars[i].widthA + sufB[i + 1];
+        const xOffset = (containerWidth - totalWidth) / 2;
+        const threshold = xOffset + prefA[i] + chars[i].widthA / 2;
+        if (sliderX > threshold) {
+            split = i + 1;
+        } else {
+            break;
+        }
+    }
+    return split;
+}
+
+/**
+ * Slices a laid-out line into three regions around a precomputed split index:
+ * a fontA bulk run, an N-char crossfade window, and a fontB bulk run.
+ *
+ * Pure and allocation-bounded: two strings plus a `windowSize`-length array per call.
+ * Takes `split` as a primitive so callers can feed it into a `$derived` and
+ * skip re-evaluation on ticks where the split index is unchanged.
+ *
+ * @param line       Line from `DualFontLayout.layout()`. Empty `chars` yields an empty model.
+ * @param split      Count of chars the slider has passed, in `[0, line.chars.length]`.
+ * @param windowSize Number of chars in the crossfade window. Clamped to `[0, line.chars.length]`.
+ *                   At line edges the window is shifted (not shrunk) to keep its size.
+ */
+export function computeLineRenderModel(
+    line: ComparisonLine,
+    split: number,
+    windowSize: number,
+): LineRenderModel {
+    const chars = line.chars;
+    const n = chars.length;
+    if (n === 0) {
+        return { leftText: '', windowChars: [], rightText: '' };
+    }
+
+    const halfWindow = Math.floor(Math.max(0, windowSize) / 2);
+    let windowStart = clamp(split - halfWindow, 0, n);
+    let windowEnd = clamp(windowStart + Math.max(0, windowSize), 0, n);
+    windowStart = Math.max(0, windowEnd - Math.max(0, windowSize));
+
+    const leftText = chars.slice(0, windowStart).map(c => c.char).join('');
+    const rightText = chars.slice(windowEnd).map(c => c.char).join('');
+    const windowChars = chars.slice(windowStart, windowEnd).map((c, idx) => ({
+        key: `${windowStart + idx}-${c.char}`,
+        char: c.char,
+        isPast: (windowStart + idx) < split,
+    }));
+
+    return { leftText, windowChars, rightText };
+}
+
+/**
+ * Clamps `value` into the inclusive range `[lo, hi]`. Assumes `lo <= hi`.
+ */
+function clamp(value: number, lo: number, hi: number): number {
+    if (value < lo) {
+        return lo;
+    }
+    if (value > hi) {
+        return hi;
+    }
+    return value;
+}

src/entities/Font/domain/index.ts

@@ -0,0 +1,11 @@
+export {
+    type ComparisonLine,
+    type ComparisonResult,
+    DualFontLayout,
+} from './DualFontLayout/DualFontLayout';
+export {
+    computeLineRenderModel,
+    findSplitIndex,
+    type LineRenderModel,
+} from './computeLineRenderModel/computeLineRenderModel';
+export { windowSizeForLine } from './windowSizeForLine/windowSizeForLine';

src/entities/Font/domain/windowSizeForLine/windowSizeForLine.test.ts

@@ -0,0 +1,38 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { windowSizeForLine } from './windowSizeForLine';
+
+describe('windowSizeForLine', () => {
+    it('returns 0 for an empty or non-positive line', () => {
+        expect(windowSizeForLine(0)).toBe(0);
+        expect(windowSizeForLine(-3)).toBe(0);
+    });
+
+    it('floors non-empty short lines at the minimum window of 1', () => {
+        expect(windowSizeForLine(1)).toBe(1);
+        expect(windowSizeForLine(2)).toBe(1);
+        expect(windowSizeForLine(3)).toBe(1);
+    });
+
+    it('scales with round(n / 3) in the mid range', () => {
+        expect(windowSizeForLine(6)).toBe(2);
+        expect(windowSizeForLine(12)).toBe(4);
+    });
+
+    it('caps at the maximum window of 5', () => {
+        expect(windowSizeForLine(15)).toBe(5);
+        expect(windowSizeForLine(16)).toBe(5);
+        expect(windowSizeForLine(100)).toBe(5);
+    });
+
+    it('rounds to nearest at fractional boundaries', () => {
+        // round(4/3)=1, round(5/3)=2, round(13/3)=4, round(14/3)=5
+        expect(windowSizeForLine(4)).toBe(1);
+        expect(windowSizeForLine(5)).toBe(2);
+        expect(windowSizeForLine(13)).toBe(4);
+        expect(windowSizeForLine(14)).toBe(5);
+    });
+});

src/entities/Font/domain/windowSizeForLine/windowSizeForLine.ts

@@ -0,0 +1,39 @@
+/**
+ * Crossfade-window sizing policy for the dual-font slider.
+ *
+ * The slider renders a band of per-char `Character` cells that opacity-crossfade
+ * between the two fonts; everything outside the band is committed native bulk
+ * text. A fixed band looked wrong on short lines — a 6-grapheme line left almost
+ * no bulk, so nearly the whole line shimmered as per-char DOM. The band size
+ * therefore scales with the line's grapheme count and caps so long lines don't
+ * pay for an oversized per-char DOM band.
+ */
+
+/**
+ * Fraction of a line's graphemes that sit in the crossfade band.
+ */
+const WINDOW_RATIO = 1 / 3;
+/**
+ * Smallest band for a non-empty line — guarantees at least one crossfading char.
+ *
+ * Accepted tradeoff: short lines now get a band of 1–2, so a fast slider drag
+ * can unmount a char before its ~100ms opacity crossfade finishes, a slight pop.
+ * Worth it for the "bulk committed, small band shimmering" look on short lines;
+ * raising this trades that pop back for less committed bulk.
+ */
+const WINDOW_MIN = 1;
+/**
+ * Largest band regardless of line length — bounds per-char DOM cost.
+ */
+const WINDOW_MAX = 5;
+
+/**
+ * Crossfade window size, in graphemes, for a line of `n` graphemes.
+ * `clamp(round(n / 3), 1, 5)`; an empty/non-positive line gets no window.
+ */
+export function windowSizeForLine(n: number): number {
+    if (n <= 0) {
+        return 0;
+    }
+    return Math.min(WINDOW_MAX, Math.max(WINDOW_MIN, Math.round(n * WINDOW_RATIO)));
+}

src/entities/Font/index.ts

@@ -1,87 +1,93 @@
-// Proxy API (PRIMARY)
 export {
-    fetchFontsByIds,
-    fetchProxyFontById,
-    fetchProxyFonts,
-} from './api/proxy/proxyFonts';
+    computeLineRenderModel,
+    DualFontLayout,
+    findSplitIndex,
+    windowSizeForLine,
+} from './domain';
 export type {
-    ProxyFontsParams,
-    ProxyFontsResponse,
-} from './api/proxy/proxyFonts';
+    ComparisonLine,
+    ComparisonResult,
+    LineRenderModel,
+} from './domain';
 
-// Fontshare API (DEPRECATED)
 export {
-    fetchAllFontshareFonts,
-    fetchFontshareFontBySlug,
-    fetchFontshareFonts,
-} from './api/fontshare/fontshare';
-export type {
-    FontshareParams,
-    FontshareResponse,
-} from './api/fontshare/fontshare';
+    createFontRowSizeResolver,
+    FontNetworkError,
+    FontResponseError,
+    getFontUrl,
+} from './lib';
+export type { FontRowSizeResolverOptions } from './lib';
 
-// Google Fonts API (DEPRECATED)
 export {
-    fetchGoogleFontFamily,
-    fetchGoogleFonts,
-} from './api/google/googleFonts';
-export type {
-    GoogleFontItem,
-    GoogleFontsParams,
-    GoogleFontsResponse,
-} from './api/google/googleFonts';
+    FontApplicator,
+    FontSampler,
+    FontVirtualList,
+} from './ui';
+
+// Pure model surface (types + constants).
 export {
-    normalizeFontshareFont,
-    normalizeFontshareFonts,
-    normalizeGoogleFont,
-    normalizeGoogleFonts,
-} from './lib/normalize/normalize';
+    DEFAULT_FONT_SIZE,
+    DEFAULT_FONT_WEIGHT,
+    DEFAULT_LETTER_SPACING,
+    DEFAULT_LINE_HEIGHT,
+    FONT_SIZE_STEP,
+    FONT_WEIGHT_STEP,
+    LETTER_SPACING_STEP,
+    LINE_HEIGHT_STEP,
+    MAX_FONT_SIZE,
+    MAX_FONT_WEIGHT,
+    MAX_LETTER_SPACING,
+    MAX_LINE_HEIGHT,
+    MIN_FONT_SIZE,
+    MIN_FONT_WEIGHT,
+    MIN_LETTER_SPACING,
+    MIN_LINE_HEIGHT,
+    VIRTUAL_INDEX_NOT_LOADED,
+} from './model/const/const';
 export type {
-    // Domain types
+    FilterGroup,
+    FilterType,
     FontCategory,
     FontCollectionFilters,
     FontCollectionSort,
-    // Store types
     FontCollectionState,
     FontFeatures,
-    FontFiles,
-    FontItem,
+    FontFilters,
+    FontLoadRequestConfig,
+    FontLoadStatus,
     FontMetadata,
     FontProvider,
-    // Fontshare API types
-    FontshareApiModel,
-    FontshareAxis,
-    FontshareDesigner,
-    FontshareFeature,
-    FontshareFont,
-    FontshareLink,
-    FontsharePublisher,
-    FontshareStyle,
-    FontshareStyleProperties,
-    FontshareTag,
-    FontshareWeight,
     FontStyleUrls,
     FontSubset,
     FontVariant,
     FontWeight,
     FontWeightItalic,
-    // Google Fonts API types
-    GoogleFontsApiModel,
-    // Normalization types
     UnifiedFont,
     UnifiedFontVariant,
-} from './model';
+} from './model/types';
 
+/*
+ * Stores are exposed as lazy accessors / classes (not eager singletons): the
+ * entity's public API is complete, so consumers go through this barrel instead
+ * of deep-importing `./model` (FSD public-API boundary). Construction happens on
+ * first call, so this is inert at import. The slice root already transitively
+ * loads `@tanstack/query-core` via `./ui` (FontVirtualList), so surfacing the
+ * stores here adds no new eager cost.
+ */
 export {
-    appliedFontsManager,
-    createUnifiedFontStore,
-    selectedFontsStore,
-    unifiedFontStore,
+    FontLifecycleManager,
+    FontsByIdsStore,
+    getFontCatalog,
+    getFontLifecycleManager,
 } from './model';
+export type { FontCatalogStore } from './model';
 
-// UI elements
-export {
-    FontApplicator,
-    FontListItem,
-    FontVirtualList,
-} from './ui';
+/*
+ * `./api` (proxy clients: `fetchProxyFonts`, `seedFontCache`, …) is intentionally
+ * NOT re-exported here — those are not part of the entity's consumed surface and
+ * importing them eagerly constructs the TanStack `queryClient`. Import via the
+ * segment: `import { fetchProxyFonts } from '$entities/Font/api'`.
+ */
+
+// `./testing` is intentionally not re-exported: fixtures must not leak into the
+// production public API. Import them via `$entities/Font/testing`.

src/entities/Font/lib/createFontLoadRequestContfig/createFontLoadRequestContfig.test.ts

@@ -0,0 +1,111 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import type { UnifiedFont } from '../../model/types';
+import { createFontLoadRequestContfig } from './createFontLoadRequestContfig';
+
+/**
+ * Minimal UnifiedFont mock — override only the fields a case exercises.
+ */
+function createMockFont(overrides: Partial<UnifiedFont> = {}): UnifiedFont {
+    const baseFont: UnifiedFont = {
+        id: 'test-font',
+        name: 'Test Font',
+        provider: 'google',
+        category: 'sans-serif',
+        subsets: ['latin'],
+        variants: [],
+        styles: {},
+        metadata: {
+            cachedAt: Date.now(),
+        },
+        features: {
+            isVariable: false,
+            tags: [],
+        },
+    };
+
+    return { ...baseFont, ...overrides };
+}
+
+describe('createFontLoadRequestContfig', () => {
+    it('builds a single-element config when a URL resolves', () => {
+        const font = createMockFont({
+            id: 'roboto',
+            name: 'Roboto',
+            styles: { variants: { '400': 'https://example.com/roboto-400.woff2' } },
+        });
+
+        const result = createFontLoadRequestContfig(font, 400);
+
+        expect(result).toEqual([
+            {
+                id: 'roboto',
+                name: 'Roboto',
+                weight: 400,
+                url: 'https://example.com/roboto-400.woff2',
+                isVariable: false,
+            },
+        ]);
+    });
+
+    it('returns an empty array when no URL resolves (flatMap drops the font)', () => {
+        const font = createMockFont({ styles: {} });
+
+        expect(createFontLoadRequestContfig(font, 400)).toEqual([]);
+    });
+
+    it('forwards isVariable from font features', () => {
+        const font = createMockFont({
+            features: { isVariable: true, tags: [] },
+            styles: { variants: { '700': 'https://example.com/inter-vf.woff2' } },
+        });
+
+        const [config] = createFontLoadRequestContfig(font, 700);
+
+        expect(config.isVariable).toBe(true);
+    });
+
+    it('sets isVariable to undefined when features is absent', () => {
+        // features is non-optional on UnifiedFont, but upstream data can be partial —
+        // the optional chain must not throw, and isVariable stays undefined.
+        const font = createMockFont({
+            styles: { variants: { '400': 'https://example.com/font.woff2' } },
+        });
+        // @ts-expect-error — deliberately drop the guaranteed field to exercise the optional chain
+        font.features = undefined;
+
+        const [config] = createFontLoadRequestContfig(font, 400);
+
+        expect(config.isVariable).toBeUndefined();
+    });
+
+    it('uses the resolved fallback URL, not just exact matches', () => {
+        // getFontUrl falls back to styles.regular when the exact weight is missing;
+        // the config must carry whatever URL actually resolved.
+        const font = createMockFont({
+            styles: { regular: 'https://example.com/font-regular.woff2' },
+        });
+
+        const [config] = createFontLoadRequestContfig(font, 900);
+
+        expect(config.url).toBe('https://example.com/font-regular.woff2');
+        expect(config.weight).toBe(900);
+    });
+
+    it('carries the requested weight even when the URL is a shared fallback', () => {
+        const font = createMockFont({
+            styles: { variants: { '400': 'https://example.com/shared.woff2' } },
+        });
+
+        expect(createFontLoadRequestContfig(font, 700)[0].weight).toBe(700);
+    });
+
+    it('propagates the invalid-weight error from getFontUrl', () => {
+        const font = createMockFont();
+
+        expect(() => createFontLoadRequestContfig(font, 450)).toThrow('Invalid weight: 450');
+    });
+});

src/entities/Font/lib/createFontLoadRequestContfig/createFontLoadRequestContfig.ts

@@ -0,0 +1,33 @@
+import type {
+    FontLoadRequestConfig,
+    UnifiedFont,
+} from '../../model';
+import { getFontUrl } from '../getFontUrl/getFontUrl';
+
+/**
+ * Build the font-lifecycle load request for a single font at a given weight.
+ *
+ * Returns a 0-or-1 element array rather than `FontLoadRequestConfig | undefined`
+ * so call sites can `flatMap` over a font list — resolve the URL and drop fonts
+ * that have none in a single pass, with no separate filter step. An empty array
+ * means the font has no loadable asset for this weight (or its fallbacks) and is
+ * silently skipped.
+ *
+ * `isVariable` is forwarded from the font's features so the lifecycle manager can
+ * dedupe variable fonts per ID (they load once regardless of weight) while still
+ * loading static fonts per weight.
+ *
+ * @param font - Unified font to load
+ * @param weight - Numeric weight (100-900)
+ * @returns Single-element config array, or `[]` when no URL resolves
+ * @throws Error when weight is outside the valid 100-900 range (propagated from `getFontUrl`)
+ */
+export function createFontLoadRequestContfig(font: UnifiedFont, weight: number): FontLoadRequestConfig[] {
+    const url = getFontUrl(font, weight);
+
+    if (!url) {
+        return [];
+    }
+
+    return [{ id: font.id, name: font.name, weight, url, isVariable: font.features?.isVariable }];
+}

src/entities/Font/lib/errors/errors.test.ts

@@ -0,0 +1,51 @@
+import {
+    FontNetworkError,
+    FontResponseError,
+} from './errors';
+
+describe('FontNetworkError', () => {
+    it('has correct name', () => {
+        const err = new FontNetworkError();
+        expect(err.name).toBe('FontNetworkError');
+    });
+
+    it('is instance of Error', () => {
+        expect(new FontNetworkError()).toBeInstanceOf(Error);
+    });
+
+    it('stores cause', () => {
+        const cause = new Error('network down');
+        const err = new FontNetworkError(cause);
+        expect(err.cause).toBe(cause);
+    });
+
+    it('has default message', () => {
+        expect(new FontNetworkError().message).toBe('Failed to fetch fonts from proxy API');
+    });
+});
+
+describe('FontResponseError', () => {
+    it('has correct name', () => {
+        const err = new FontResponseError('response', undefined);
+        expect(err.name).toBe('FontResponseError');
+    });
+
+    it('is instance of Error', () => {
+        expect(new FontResponseError('response.fonts', null)).toBeInstanceOf(Error);
+    });
+
+    it('stores field', () => {
+        const err = new FontResponseError('response.fonts', 42);
+        expect(err.field).toBe('response.fonts');
+    });
+
+    it('stores received value', () => {
+        const err = new FontResponseError('response.fonts', 42);
+        expect(err.received).toBe(42);
+    });
+
+    it('message includes field name', () => {
+        const err = new FontResponseError('response.fonts', null);
+        expect(err.message).toContain('response.fonts');
+    });
+});

src/entities/Font/lib/errors/errors.ts

@@ -0,0 +1,32 @@
+import { NonRetryableError } from '$shared/api/nonRetryableError';
+
+/**
+ * Thrown when the network request to the proxy API fails.
+ * Wraps the underlying fetch error (timeout, DNS failure, connection refused, etc.).
+ */
+export class FontNetworkError extends Error {
+    readonly name = 'FontNetworkError';
+
+    constructor(public readonly cause?: unknown) {
+        super('Failed to fetch fonts from proxy API');
+    }
+}
+
+/**
+ * Thrown when the proxy API returns a response with an unexpected shape.
+ * Extends NonRetryableError because schema mismatches are not transient —
+ * retrying will produce the same failure and only delay surfacing the bug.
+ *
+ * @property field - The name of the field that failed validation (e.g. `'response'`, `'response.fonts'`).
+ * @property received - The actual value received at that field, for debugging.
+ */
+export class FontResponseError extends NonRetryableError {
+    readonly name = 'FontResponseError';
+
+    constructor(
+        public readonly field: string,
+        public readonly received: unknown,
+    ) {
+        super(`Invalid proxy API response: ${field}`);
+    }
+}

src/entities/Font/lib/getFontUrl/getFontUrl.ts

@@ -3,13 +3,33 @@ import type {
     UnifiedFont,
 } from '../../model';
 
+/**
+ * Valid font weight values (100-900 in increments of 100)
+ */
 const SIZES = [100, 200, 300, 400, 500, 600, 700, 800, 900];
 
 /**
- * Constructs a URL for a font based on the provided font and weight.
- * @param font - The font object.
- * @param weight - The weight of the font.
- * @returns The URL for the font.
+ * Gets the URL for a font file at a specific weight
+ *
+ * Constructs the appropriate URL for loading a font file based on
+ * the font object and requested weight. Handles variable fonts and
+ * provides fallbacks for static fonts.
+ *
+ * @param font - Unified font object containing style URLs
+ * @param weight - Font weight (100-900)
+ * @returns URL string for the font file, or undefined if not found
+ * @throws Error if weight is not a valid value (100-900)
+ *
+ * @example
+ * ```ts
+ * const url = getFontUrl(roboto, 700); // Returns URL for Roboto Bold
+ *
+ * // Variable fonts: backend maps weight to VF URL
+ * const vfUrl = getFontUrl(inter, 450); // Returns variable font URL
+ *
+ * // Fallback for missing weights
+ * const fallback = getFontUrl(font, 900); // Falls back to regular/400 if 900 missing
+ * ```
  */
 export function getFontUrl(font: UnifiedFont, weight: number): string | undefined {
     if (!SIZES.includes(weight)) {
@@ -18,12 +38,11 @@ export function getFontUrl(font: UnifiedFont, weight: number): string | undefine
 
     const weightKey = weight.toString() as FontWeight;
 
-    // 1. Try exact match (Backend now maps "100".."900" to VF URL if variable)
+    // Try exact match (backend maps weight to VF URL for variable fonts)
     if (font.styles.variants?.[weightKey]) {
         return font.styles.variants[weightKey];
     }
 
-    // 2. Fallbacks for Static Fonts (if exact weight missing)
-    // Try 'regular' or '400' as safe defaults
+    // Fallbacks for static fonts when exact weight is missing
     return font.styles.regular || font.styles.variants?.['400'] || font.styles.variants?.['regular'];
 }

src/entities/Font/lib/index.ts

@@ -1,8 +1,9 @@
-export {
-    normalizeFontshareFont,
-    normalizeFontshareFonts,
-    normalizeGoogleFont,
-    normalizeGoogleFonts,
-} from './normalize/normalize';
-
 export { getFontUrl } from './getFontUrl/getFontUrl';
+
+export {
+    FontNetworkError,
+    FontResponseError,
+} from './errors/errors';
+
+export { createFontRowSizeResolver } from './sizeResolver/createFontRowSizeResolver';
+export type { FontRowSizeResolverOptions } from './sizeResolver/createFontRowSizeResolver';

src/entities/Font/lib/normalize/normalize.test.ts

@@ -1,582 +0,0 @@
-import {
-    describe,
-    expect,
-    it,
-} from 'vitest';
-import type {
-    FontItem,
-    FontshareFont,
-    GoogleFontItem,
-} from '../../model/types';
-import {
-    normalizeFontshareFont,
-    normalizeFontshareFonts,
-    normalizeGoogleFont,
-    normalizeGoogleFonts,
-} from './normalize';
-
-describe('Font Normalization', () => {
-    describe('normalizeGoogleFont', () => {
-        const mockGoogleFont: GoogleFontItem = {
-            family: 'Roboto',
-            category: 'sans-serif',
-            variants: ['regular', '700', 'italic', '700italic'],
-            subsets: ['latin', 'latin-ext'],
-            files: {
-                regular: 'https://fonts.gstatic.com/s/roboto/v30/KFOmCnqEu92Fr1Mu72xKOzY.woff2',
-                '700': 'https://fonts.gstatic.com/s/roboto/v30/KFOlCnqEu92Fr1Mu72xWUlvAx05IsDqlA.woff2',
-                italic: 'https://fonts.gstatic.com/s/roboto/v30/KFOkCnqEu92Fr1Mu51xIIzI.woff2',
-                '700italic': 'https://fonts.gstatic.com/s/roboto/v30/KFOjCnqEu92Fr1Mu51TzBic6CsQ.woff2',
-            },
-            version: 'v30',
-            lastModified: '2022-01-01',
-            menu: 'https://fonts.googleapis.com/css2?family=Roboto',
-        };
-
-        it('normalizes Google Font to unified model', () => {
-            const result = normalizeGoogleFont(mockGoogleFont);
-
-            expect(result.id).toBe('Roboto');
-            expect(result.name).toBe('Roboto');
-            expect(result.provider).toBe('google');
-            expect(result.category).toBe('sans-serif');
-        });
-
-        it('maps font variants correctly', () => {
-            const result = normalizeGoogleFont(mockGoogleFont);
-
-            expect(result.variants).toEqual(['regular', '700', 'italic', '700italic']);
-        });
-
-        it('maps subsets correctly', () => {
-            const result = normalizeGoogleFont(mockGoogleFont);
-
-            expect(result.subsets).toContain('latin');
-            expect(result.subsets).toContain('latin-ext');
-            expect(result.subsets).toHaveLength(2);
-        });
-
-        it('maps style URLs correctly', () => {
-            const result = normalizeGoogleFont(mockGoogleFont);
-
-            expect(result.styles.regular).toBeDefined();
-            expect(result.styles.bold).toBeDefined();
-            expect(result.styles.italic).toBeDefined();
-            expect(result.styles.boldItalic).toBeDefined();
-        });
-
-        it('includes metadata', () => {
-            const result = normalizeGoogleFont(mockGoogleFont);
-
-            expect(result.metadata.cachedAt).toBeDefined();
-            expect(result.metadata.version).toBe('v30');
-            expect(result.metadata.lastModified).toBe('2022-01-01');
-        });
-
-        it('marks Google Fonts as non-variable', () => {
-            const result = normalizeGoogleFont(mockGoogleFont);
-
-            expect(result.features.isVariable).toBe(false);
-            expect(result.features.tags).toEqual([]);
-        });
-
-        it('handles sans-serif category', () => {
-            const font: FontItem = { ...mockGoogleFont, category: 'sans-serif' };
-            const result = normalizeGoogleFont(font);
-
-            expect(result.category).toBe('sans-serif');
-        });
-
-        it('handles serif category', () => {
-            const font: FontItem = { ...mockGoogleFont, category: 'serif' };
-            const result = normalizeGoogleFont(font);
-
-            expect(result.category).toBe('serif');
-        });
-
-        it('handles display category', () => {
-            const font: FontItem = { ...mockGoogleFont, category: 'display' };
-            const result = normalizeGoogleFont(font);
-
-            expect(result.category).toBe('display');
-        });
-
-        it('handles handwriting category', () => {
-            const font: FontItem = { ...mockGoogleFont, category: 'handwriting' };
-            const result = normalizeGoogleFont(font);
-
-            expect(result.category).toBe('handwriting');
-        });
-
-        it('handles cursive category (maps to handwriting)', () => {
-            const font: FontItem = { ...mockGoogleFont, category: 'cursive' as any };
-            const result = normalizeGoogleFont(font);
-
-            expect(result.category).toBe('handwriting');
-        });
-
-        it('handles monospace category', () => {
-            const font: FontItem = { ...mockGoogleFont, category: 'monospace' };
-            const result = normalizeGoogleFont(font);
-
-            expect(result.category).toBe('monospace');
-        });
-
-        it('filters invalid subsets', () => {
-            const font = {
-                ...mockGoogleFont,
-                subsets: ['latin', 'latin-ext', 'invalid-subset'],
-            };
-            const result = normalizeGoogleFont(font);
-
-            expect(result.subsets).not.toContain('invalid-subset');
-            expect(result.subsets).toHaveLength(2);
-        });
-
-        it('maps variant weights correctly', () => {
-            const font: GoogleFontItem = {
-                ...mockGoogleFont,
-                variants: ['regular', '100', '400', '700', '900'] as any,
-            };
-            const result = normalizeGoogleFont(font);
-
-            expect(result.variants).toContain('regular');
-            expect(result.variants).toContain('100');
-            expect(result.variants).toContain('400');
-            expect(result.variants).toContain('700');
-            expect(result.variants).toContain('900');
-        });
-    });
-
-    describe('normalizeFontshareFont', () => {
-        const mockFontshareFont: FontshareFont = {
-            id: '20e9fcdc-1e41-4559-a43d-1ede0adc8896',
-            name: 'Satoshi',
-            native_name: null,
-            slug: 'satoshi',
-            category: 'Sans',
-            script: 'latin',
-            publisher: {
-                bio: 'Indian Type Foundry',
-                email: null,
-                id: 'test-id',
-                links: [],
-                name: 'Indian Type Foundry',
-            },
-            designers: [
-                {
-                    bio: 'Designer bio',
-                    links: [],
-                    name: 'Designer Name',
-                },
-            ],
-            related_families: null,
-            display_publisher_as_designer: false,
-            trials_enabled: true,
-            show_latin_metrics: false,
-            license_type: 'itf_ffl',
-            languages: 'Afar, Afrikaans',
-            inserted_at: '2021-03-12T20:49:05Z',
-            story: '<p>Font story</p>',
-            version: '1.0',
-            views: 10000,
-            views_recent: 500,
-            is_hot: true,
-            is_new: false,
-            is_shortlisted: false,
-            is_top: true,
-            axes: [],
-            font_tags: [
-                { name: 'Branding' },
-                { name: 'Logos' },
-            ],
-            features: [
-                {
-                    name: 'Alternate t',
-                    on_by_default: false,
-                    tag: 'ss01',
-                },
-            ],
-            styles: [
-                {
-                    id: 'style-id-1',
-                    default: true,
-                    file: '//cdn.fontshare.com/wf/satoshi.woff2',
-                    is_italic: false,
-                    is_variable: false,
-                    properties: {},
-                    weight: {
-                        label: 'Regular',
-                        name: 'Regular',
-                        native_name: null,
-                        number: 400,
-                        weight: 400,
-                    },
-                },
-                {
-                    id: 'style-id-2',
-                    default: false,
-                    file: '//cdn.fontshare.com/wf/satoshi-bold.woff2',
-                    is_italic: false,
-                    is_variable: false,
-                    properties: {},
-                    weight: {
-                        label: 'Bold',
-                        name: 'Bold',
-                        native_name: null,
-                        number: 700,
-                        weight: 700,
-                    },
-                },
-                {
-                    id: 'style-id-3',
-                    default: false,
-                    file: '//cdn.fontshare.com/wf/satoshi-italic.woff2',
-                    is_italic: true,
-                    is_variable: false,
-                    properties: {},
-                    weight: {
-                        label: 'Regular',
-                        name: 'Regular',
-                        native_name: null,
-                        number: 400,
-                        weight: 400,
-                    },
-                },
-                {
-                    id: 'style-id-4',
-                    default: false,
-                    file: '//cdn.fontshare.com/wf/satoshi-bolditalic.woff2',
-                    is_italic: true,
-                    is_variable: false,
-                    properties: {},
-                    weight: {
-                        label: 'Bold',
-                        name: 'Bold',
-                        native_name: null,
-                        number: 700,
-                        weight: 700,
-                    },
-                },
-            ],
-        };
-
-        it('normalizes Fontshare font to unified model', () => {
-            const result = normalizeFontshareFont(mockFontshareFont);
-
-            expect(result.id).toBe('satoshi');
-            expect(result.name).toBe('Satoshi');
-            expect(result.provider).toBe('fontshare');
-            expect(result.category).toBe('sans-serif');
-        });
-
-        it('uses slug as unique identifier', () => {
-            const result = normalizeFontshareFont(mockFontshareFont);
-
-            expect(result.id).toBe('satoshi');
-        });
-
-        it('extracts variant names from styles', () => {
-            const result = normalizeFontshareFont(mockFontshareFont);
-
-            expect(result.variants).toContain('Regular');
-            expect(result.variants).toContain('Bold');
-            expect(result.variants).toContain('Regularitalic');
-            expect(result.variants).toContain('Bolditalic');
-        });
-
-        it('maps Fontshare Sans to sans-serif category', () => {
-            const font = { ...mockFontshareFont, category: 'Sans' };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.category).toBe('sans-serif');
-        });
-
-        it('maps Fontshare Serif to serif category', () => {
-            const font = { ...mockFontshareFont, category: 'Serif' };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.category).toBe('serif');
-        });
-
-        it('maps Fontshare Display to display category', () => {
-            const font = { ...mockFontshareFont, category: 'Display' };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.category).toBe('display');
-        });
-
-        it('maps Fontshare Script to handwriting category', () => {
-            const font = { ...mockFontshareFont, category: 'Script' };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.category).toBe('handwriting');
-        });
-
-        it('maps Fontshare Mono to monospace category', () => {
-            const font = { ...mockFontshareFont, category: 'Mono' };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.category).toBe('monospace');
-        });
-
-        it('maps style URLs correctly', () => {
-            const result = normalizeFontshareFont(mockFontshareFont);
-
-            expect(result.styles.regular).toBe('//cdn.fontshare.com/wf/satoshi.woff2');
-            expect(result.styles.bold).toBe('//cdn.fontshare.com/wf/satoshi-bold.woff2');
-            expect(result.styles.italic).toBe('//cdn.fontshare.com/wf/satoshi-italic.woff2');
-            expect(result.styles.boldItalic).toBe(
-                '//cdn.fontshare.com/wf/satoshi-bolditalic.woff2',
-            );
-        });
-
-        it('handles variable fonts', () => {
-            const variableFont: FontshareFont = {
-                ...mockFontshareFont,
-                axes: [
-                    {
-                        name: 'wght',
-                        property: 'wght',
-                        range_default: 400,
-                        range_left: 300,
-                        range_right: 900,
-                    },
-                ],
-                styles: [
-                    {
-                        id: 'var-style',
-                        default: true,
-                        file: '//cdn.fontshare.com/wf/satoshi-variable.woff2',
-                        is_italic: false,
-                        is_variable: true,
-                        properties: {},
-                        weight: {
-                            label: 'Variable',
-                            name: 'Variable',
-                            native_name: null,
-                            number: 0,
-                            weight: 0,
-                        },
-                    },
-                ],
-            };
-
-            const result = normalizeFontshareFont(variableFont);
-
-            expect(result.features.isVariable).toBe(true);
-            expect(result.features.axes).toHaveLength(1);
-            expect(result.features.axes?.[0].name).toBe('wght');
-        });
-
-        it('extracts font tags', () => {
-            const result = normalizeFontshareFont(mockFontshareFont);
-
-            expect(result.features.tags).toContain('Branding');
-            expect(result.features.tags).toContain('Logos');
-            expect(result.features.tags).toHaveLength(2);
-        });
-
-        it('includes popularity from views', () => {
-            const result = normalizeFontshareFont(mockFontshareFont);
-
-            expect(result.metadata.popularity).toBe(10000);
-        });
-
-        it('includes metadata', () => {
-            const result = normalizeFontshareFont(mockFontshareFont);
-
-            expect(result.metadata.cachedAt).toBeDefined();
-            expect(result.metadata.version).toBe('1.0');
-            expect(result.metadata.lastModified).toBe('2021-03-12T20:49:05Z');
-        });
-
-        it('handles missing subsets gracefully', () => {
-            const font = {
-                ...mockFontshareFont,
-                script: 'invalid-script',
-            };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.subsets).toEqual([]);
-        });
-
-        it('handles empty tags', () => {
-            const font = {
-                ...mockFontshareFont,
-                font_tags: [],
-            };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.features.tags).toBeUndefined();
-        });
-
-        it('handles empty axes', () => {
-            const font = {
-                ...mockFontshareFont,
-                axes: [],
-            };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.features.isVariable).toBe(false);
-            expect(result.features.axes).toBeUndefined();
-        });
-    });
-
-    describe('normalizeGoogleFonts', () => {
-        it('normalizes array of Google Fonts', () => {
-            const fonts: GoogleFontItem[] = [
-                {
-                    family: 'Roboto',
-                    category: 'sans-serif',
-                    variants: ['regular'],
-                    subsets: ['latin'],
-                    files: { regular: 'url' },
-                    version: 'v1',
-                    lastModified: '2022-01-01',
-                    menu: 'https://fonts.googleapis.com/css2?family=Roboto',
-                },
-                {
-                    family: 'Open Sans',
-                    category: 'sans-serif',
-                    variants: ['regular'],
-                    subsets: ['latin'],
-                    files: { regular: 'url' },
-                    version: 'v1',
-                    lastModified: '2022-01-01',
-                    menu: 'https://fonts.googleapis.com/css2?family=Open+Sans',
-                },
-            ];
-
-            const result = normalizeGoogleFonts(fonts);
-
-            expect(result).toHaveLength(2);
-            expect(result[0].name).toBe('Roboto');
-            expect(result[1].name).toBe('Open Sans');
-        });
-
-        it('returns empty array for empty input', () => {
-            const result = normalizeGoogleFonts([]);
-
-            expect(result).toEqual([]);
-        });
-    });
-
-    describe('normalizeFontshareFonts', () => {
-        it('normalizes array of Fontshare fonts', () => {
-            const fonts: FontshareFont[] = [
-                {
-                    ...mockMinimalFontshareFont('font1', 'Font 1'),
-                },
-                {
-                    ...mockMinimalFontshareFont('font2', 'Font 2'),
-                },
-            ];
-
-            const result = normalizeFontshareFonts(fonts);
-
-            expect(result).toHaveLength(2);
-            expect(result[0].name).toBe('Font 1');
-            expect(result[1].name).toBe('Font 2');
-        });
-
-        it('returns empty array for empty input', () => {
-            const result = normalizeFontshareFonts([]);
-
-            expect(result).toEqual([]);
-        });
-    });
-
-    describe('edge cases', () => {
-        it('handles Google Font with missing optional fields', () => {
-            const font: Partial<GoogleFontItem> = {
-                family: 'Test Font',
-                category: 'sans-serif',
-                variants: ['regular'],
-                subsets: ['latin'],
-                files: { regular: 'url' },
-            };
-
-            const result = normalizeGoogleFont(font as GoogleFontItem);
-
-            expect(result.id).toBe('Test Font');
-            expect(result.metadata.version).toBeUndefined();
-            expect(result.metadata.lastModified).toBeUndefined();
-        });
-
-        it('handles Fontshare font with minimal data', () => {
-            const result = normalizeFontshareFont(mockMinimalFontshareFont('slug', 'Name'));
-
-            expect(result.id).toBe('slug');
-            expect(result.name).toBe('Name');
-            expect(result.provider).toBe('fontshare');
-        });
-
-        it('handles unknown Fontshare category', () => {
-            const font = {
-                ...mockMinimalFontshareFont('slug', 'Name'),
-                category: 'Unknown Category',
-            };
-            const result = normalizeFontshareFont(font);
-
-            expect(result.category).toBe('sans-serif'); // fallback
-        });
-    });
-});
-
-/**
- * Helper function to create minimal Fontshare font mock
- */
-function mockMinimalFontshareFont(slug: string, name: string): FontshareFont {
-    return {
-        id: 'test-id',
-        name,
-        native_name: null,
-        slug,
-        category: 'Sans',
-        script: 'latin',
-        publisher: {
-            bio: '',
-            email: null,
-            id: '',
-            links: [],
-            name: '',
-        },
-        designers: [],
-        related_families: null,
-        display_publisher_as_designer: false,
-        trials_enabled: false,
-        show_latin_metrics: false,
-        license_type: '',
-        languages: '',
-        inserted_at: '',
-        story: '',
-        version: '1.0',
-        views: 0,
-        views_recent: 0,
-        is_hot: false,
-        is_new: false,
-        is_shortlisted: null,
-        is_top: false,
-        axes: [],
-        font_tags: [],
-        features: [],
-        styles: [
-            {
-                id: 'style-id',
-                default: true,
-                file: '//cdn.fontshare.com/wf/test.woff2',
-                is_italic: false,
-                is_variable: false,
-                properties: {},
-                weight: {
-                    label: 'Regular',
-                    name: 'Regular',
-                    native_name: null,
-                    number: 400,
-                    weight: 400,
-                },
-            },
-        ],
-    };
-}

src/entities/Font/lib/normalize/normalize.ts

@@ -1,275 +0,0 @@
-/**
- * Normalize fonts from Google Fonts and Fontshare to unified model
- *
- * Transforms provider-specific font data into a common interface
- * for consistent handling across the application.
- */
-
-import type {
-    FontCategory,
-    FontStyleUrls,
-    FontSubset,
-    FontshareFont,
-    GoogleFontItem,
-    UnifiedFont,
-    UnifiedFontVariant,
-} from '../../model/types';
-
-/**
- * Map Google Fonts category to unified FontCategory
- */
-function mapGoogleCategory(category: string): FontCategory {
-    const normalized = category.toLowerCase();
-    if (normalized.includes('sans-serif')) {
-        return 'sans-serif';
-    }
-    if (normalized.includes('serif')) {
-        return 'serif';
-    }
-    if (normalized.includes('display')) {
-        return 'display';
-    }
-    if (normalized.includes('handwriting') || normalized.includes('cursive')) {
-        return 'handwriting';
-    }
-    if (normalized.includes('monospace')) {
-        return 'monospace';
-    }
-    // Default fallback
-    return 'sans-serif';
-}
-
-/**
- * Map Fontshare category to unified FontCategory
- */
-function mapFontshareCategory(category: string): FontCategory {
-    const normalized = category.toLowerCase();
-    if (normalized === 'sans' || normalized === 'sans-serif') {
-        return 'sans-serif';
-    }
-    if (normalized === 'serif') {
-        return 'serif';
-    }
-    if (normalized === 'display') {
-        return 'display';
-    }
-    if (normalized === 'script') {
-        return 'handwriting';
-    }
-    if (normalized === 'mono' || normalized === 'monospace') {
-        return 'monospace';
-    }
-    // Default fallback
-    return 'sans-serif';
-}
-
-/**
- * Map Google subset to unified FontSubset
- */
-function mapGoogleSubset(subset: string): FontSubset | null {
-    const validSubsets: FontSubset[] = [
-        'latin',
-        'latin-ext',
-        'cyrillic',
-        'greek',
-        'arabic',
-        'devanagari',
-    ];
-    return validSubsets.includes(subset as FontSubset)
-        ? (subset as FontSubset)
-        : null;
-}
-
-/**
- * Map Fontshare script to unified FontSubset
- */
-function mapFontshareScript(script: string): FontSubset | null {
-    const normalized = script.toLowerCase();
-    const mapping: Record<string, FontSubset | null> = {
-        latin: 'latin',
-        'latin-ext': 'latin-ext',
-        cyrillic: 'cyrillic',
-        greek: 'greek',
-        arabic: 'arabic',
-        devanagari: 'devanagari',
-    };
-    return mapping[normalized] ?? null;
-}
-
-/**
- * Normalize Google Font to unified model
- *
- * @param apiFont - Font item from Google Fonts API
- * @returns Unified font model
- *
- * @example
- * ```ts
- * const roboto = normalizeGoogleFont({
- *     family: 'Roboto',
- *     category: 'sans-serif',
- *     variants: ['regular', '700'],
- *     subsets: ['latin', 'latin-ext'],
- *     files: { regular: '...', '700': '...' }
- * });
- *
- * console.log(roboto.id); // 'Roboto'
- * console.log(roboto.provider); // 'google'
- * ```
- */
-export function normalizeGoogleFont(apiFont: GoogleFontItem): UnifiedFont {
-    const category = mapGoogleCategory(apiFont.category);
-    const subsets = apiFont.subsets
-        .map(mapGoogleSubset)
-        .filter((subset): subset is FontSubset => subset !== null);
-
-    // Map variant files to style URLs
-    const styles: FontStyleUrls = {};
-    for (const [variant, url] of Object.entries(apiFont.files)) {
-        const urlString = url as string; // Type assertion for Record<string, string>
-        if (variant === 'regular' || variant === '400') {
-            styles.regular = urlString;
-        } else if (variant === 'italic' || variant === '400italic') {
-            styles.italic = urlString;
-        } else if (variant === 'bold' || variant === '700') {
-            styles.bold = urlString;
-        } else if (variant === 'bolditalic' || variant === '700italic') {
-            styles.boldItalic = urlString;
-        }
-    }
-
-    return {
-        id: apiFont.family,
-        name: apiFont.family,
-        provider: 'google',
-        category,
-        subsets,
-        variants: apiFont.variants,
-        styles,
-        metadata: {
-            cachedAt: Date.now(),
-            version: apiFont.version,
-            lastModified: apiFont.lastModified,
-        },
-        features: {
-            isVariable: false, // Google Fonts doesn't expose variable font info
-            tags: [],
-        },
-    };
-}
-
-/**
- * Normalize Fontshare font to unified model
- *
- * @param apiFont - Font item from Fontshare API
- * @returns Unified font model
- *
- * @example
- * ```ts
- * const satoshi = normalizeFontshareFont({
- *     id: 'uuid',
- *     name: 'Satoshi',
- *     slug: 'satoshi',
- *     category: 'Sans',
- *     script: 'latin',
- *     styles: [ ... ]
- * });
- *
- * console.log(satoshi.id); // 'satoshi'
- * console.log(satoshi.provider); // 'fontshare'
- * ```
- */
-export function normalizeFontshareFont(apiFont: FontshareFont): UnifiedFont {
-    const category = mapFontshareCategory(apiFont.category);
-    const subset = mapFontshareScript(apiFont.script);
-    const subsets = subset ? [subset] : [];
-
-    // Extract variant names from styles
-    const variants = apiFont.styles.map(style => {
-        const weightLabel = style.weight.label;
-        const isItalic = style.is_italic;
-        return (isItalic ? `${weightLabel}italic` : weightLabel) as UnifiedFontVariant;
-    });
-
-    // Map styles to URLs
-    const styles: FontStyleUrls = {};
-    for (const style of apiFont.styles) {
-        if (style.is_variable) {
-            // Variable font - store as primary variant
-            styles.regular = style.file;
-            break;
-        }
-
-        const weight = style.weight.number;
-        const isItalic = style.is_italic;
-
-        if (weight === 400 && !isItalic) {
-            styles.regular = style.file;
-        } else if (weight === 400 && isItalic) {
-            styles.italic = style.file;
-        } else if (weight >= 700 && !isItalic) {
-            styles.bold = style.file;
-        } else if (weight >= 700 && isItalic) {
-            styles.boldItalic = style.file;
-        }
-    }
-
-    // Extract variable font axes
-    const axes = apiFont.axes.map(axis => ({
-        name: axis.name,
-        property: axis.property,
-        default: axis.range_default,
-        min: axis.range_left,
-        max: axis.range_right,
-    }));
-
-    // Extract tags
-    const tags = apiFont.font_tags.map(tag => tag.name);
-
-    return {
-        id: apiFont.slug,
-        name: apiFont.name,
-        provider: 'fontshare',
-        category,
-        subsets,
-        variants,
-        styles,
-        metadata: {
-            cachedAt: Date.now(),
-            version: apiFont.version,
-            lastModified: apiFont.inserted_at,
-            popularity: apiFont.views,
-        },
-        features: {
-            isVariable: apiFont.axes.length > 0,
-            axes: axes.length > 0 ? axes : undefined,
-            tags: tags.length > 0 ? tags : undefined,
-        },
-    };
-}
-
-/**
- * Normalize multiple Google Fonts to unified model
- *
- * @param apiFonts - Array of Google Font items
- * @returns Array of unified fonts
- */
-export function normalizeGoogleFonts(
-    apiFonts: GoogleFontItem[],
-): UnifiedFont[] {
-    return apiFonts.map(normalizeGoogleFont);
-}
-
-/**
- * Normalize multiple Fontshare fonts to unified model
- *
- * @param apiFonts - Array of Fontshare font items
- * @returns Array of unified fonts
- */
-export function normalizeFontshareFonts(
-    apiFonts: FontshareFont[],
-): UnifiedFont[] {
-    return apiFonts.map(normalizeFontshareFont);
-}
-
-// Re-export UnifiedFont for backward compatibility
-export type { UnifiedFont } from '../../model/types/normalize';

src/entities/Font/lib/sizeResolver/createFontRowSizeResolver.test.ts

@@ -0,0 +1,180 @@
+// @vitest-environment jsdom
+import { installCanvasMock } from '$shared/lib/helpers/__mocks__/canvas';
+import {
+    clearCache,
+    layout,
+} from '@chenglou/pretext';
+
+// Wrap pretext's `layout` in a spy-able mock so tests can assert call counts.
+// `vi.mock` is hoisted, so the import above receives the mocked module.
+vi.mock('@chenglou/pretext', async () => {
+    const actual = await vi.importActual<typeof import('@chenglou/pretext')>('@chenglou/pretext');
+    return {
+        ...actual,
+        layout: vi.fn(actual.layout),
+    };
+});
+import { mockUnifiedFont } from '$entities/Font/testing';
+import {
+    beforeEach,
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import type { FontLoadStatus } from '../../model/types';
+import { createFontRowSizeResolver } from './createFontRowSizeResolver';
+
+// Fixed-width canvas mock: every character is 10px wide regardless of font.
+// This makes wrapping math predictable: N chars × 10px = N×10 total width.
+const CHAR_WIDTH = 10;
+const LINE_HEIGHT = 20;
+const CONTAINER_WIDTH = 200;
+const CONTENT_PADDING_X = 32; // p-4 × 2 sides = 32px
+const CHROME_HEIGHT = 56;
+const FALLBACK_HEIGHT = 220;
+const FONT_SIZE_PX = 16;
+
+describe('createFontRowSizeResolver', () => {
+    let statusMap: Map<string, FontLoadStatus>;
+    let getStatus: (key: string) => FontLoadStatus | undefined;
+
+    beforeEach(() => {
+        installCanvasMock((_font, text) => text.length * CHAR_WIDTH);
+        clearCache();
+        statusMap = new Map();
+        getStatus = key => statusMap.get(key);
+    });
+
+    function makeResolver(overrides?: Partial<Parameters<typeof createFontRowSizeResolver>[0]>) {
+        const font = mockUnifiedFont({ id: 'inter', name: 'Inter' });
+        return {
+            font,
+            resolver: createFontRowSizeResolver({
+                getFonts: () => [font],
+                getWeight: () => 400,
+                getPreviewText: () => 'Hello',
+                getContainerWidth: () => CONTAINER_WIDTH,
+                getFontSizePx: () => FONT_SIZE_PX,
+                getLineHeightPx: () => LINE_HEIGHT,
+                getStatus,
+                contentHorizontalPadding: CONTENT_PADDING_X,
+                chromeHeight: CHROME_HEIGHT,
+                fallbackHeight: FALLBACK_HEIGHT,
+                ...overrides,
+            }),
+        };
+    }
+
+    it('returns fallbackHeight when font status is undefined', () => {
+        const { resolver } = makeResolver();
+        expect(resolver(0)).toBe(FALLBACK_HEIGHT);
+    });
+
+    it('returns fallbackHeight when font status is "loading"', () => {
+        const { resolver } = makeResolver();
+        statusMap.set('inter@400', 'loading');
+        expect(resolver(0)).toBe(FALLBACK_HEIGHT);
+    });
+
+    it('returns fallbackHeight when font status is "error"', () => {
+        const { resolver } = makeResolver();
+        statusMap.set('inter@400', 'error');
+        expect(resolver(0)).toBe(FALLBACK_HEIGHT);
+    });
+
+    it('returns fallbackHeight when containerWidth is 0', () => {
+        const { resolver } = makeResolver({ getContainerWidth: () => 0 });
+        statusMap.set('inter@400', 'loaded');
+        expect(resolver(0)).toBe(FALLBACK_HEIGHT);
+    });
+
+    it('returns fallbackHeight when previewText is empty', () => {
+        const { resolver } = makeResolver({ getPreviewText: () => '' });
+        statusMap.set('inter@400', 'loaded');
+        expect(resolver(0)).toBe(FALLBACK_HEIGHT);
+    });
+
+    it('returns fallbackHeight for out-of-bounds rowIndex', () => {
+        const { resolver } = makeResolver();
+        statusMap.set('inter@400', 'loaded');
+        expect(resolver(99)).toBe(FALLBACK_HEIGHT);
+    });
+
+    it('returns computed height (totalHeight + chromeHeight) when font is loaded', () => {
+        const { resolver } = makeResolver();
+        statusMap.set('inter@400', 'loaded');
+
+        // 'Hello' = 5 chars × 10px = 50px. contentWidth = 200 - 32 = 168px. Fits on one line.
+        // totalHeight = 1 × LINE_HEIGHT = 20. result = 20 + CHROME_HEIGHT = 76.
+        const result = resolver(0);
+        expect(result).toBe(LINE_HEIGHT + CHROME_HEIGHT);
+    });
+
+    it('returns increased height when text wraps due to narrow container', () => {
+        // contentWidth = 40 - 32 = 8px — 'Hello' (50px) forces wrapping onto many lines
+        const { resolver } = makeResolver({ getContainerWidth: () => 40 });
+        statusMap.set('inter@400', 'loaded');
+
+        const result = resolver(0);
+        expect(result).toBeGreaterThan(LINE_HEIGHT + CHROME_HEIGHT);
+    });
+
+    it('does not call layout() again on second call with same arguments', () => {
+        const { resolver } = makeResolver();
+        statusMap.set('inter@400', 'loaded');
+
+        const layoutSpy = vi.mocked(layout);
+        layoutSpy.mockClear();
+
+        resolver(0);
+        resolver(0);
+
+        expect(layoutSpy).toHaveBeenCalledTimes(1);
+    });
+
+    it('calls layout() again when containerWidth changes (cache miss)', () => {
+        let width = CONTAINER_WIDTH;
+        const { resolver } = makeResolver({ getContainerWidth: () => width });
+        statusMap.set('inter@400', 'loaded');
+
+        const layoutSpy = vi.mocked(layout);
+        layoutSpy.mockClear();
+
+        resolver(0);
+        width = 100;
+        resolver(0);
+
+        expect(layoutSpy).toHaveBeenCalledTimes(2);
+    });
+
+    it('returns greater height when container narrows (more wrapping)', () => {
+        let width = CONTAINER_WIDTH;
+        const { resolver } = makeResolver({ getContainerWidth: () => width });
+        statusMap.set('inter@400', 'loaded');
+
+        const h1 = resolver(0);
+        width = 100; // narrower → more wrapping
+        const h2 = resolver(0);
+
+        expect(h2).toBeGreaterThanOrEqual(h1);
+    });
+
+    it('uses variable font key for variable fonts', () => {
+        const vfFont = mockUnifiedFont({ id: 'roboto', name: 'Roboto', features: { isVariable: true } });
+        const { resolver } = makeResolver({ getFonts: () => [vfFont] });
+        // Variable fonts use '{id}@vf' key, not '{id}@{weight}'
+        statusMap.set('roboto@vf', 'loaded');
+        const result = resolver(0);
+        expect(result).not.toBe(FALLBACK_HEIGHT);
+        expect(result).toBeGreaterThan(0);
+    });
+
+    it('returns fallbackHeight for variable font when static key is set instead', () => {
+        const vfFont = mockUnifiedFont({ id: 'roboto', name: 'Roboto', features: { isVariable: true } });
+        const { resolver } = makeResolver({ getFonts: () => [vfFont] });
+        // Setting the static key should NOT unlock computed height for variable fonts
+        statusMap.set('roboto@400', 'loaded');
+        expect(resolver(0)).toBe(FALLBACK_HEIGHT);
+    });
+});

src/entities/Font/lib/sizeResolver/createFontRowSizeResolver.ts

@@ -0,0 +1,140 @@
+import {
+    layout,
+    prepare,
+} from '@chenglou/pretext';
+import { generateFontKey } from '../../model/store/fontLifecycleManager/utils/generateFontKey/generateFontKey';
+import type {
+    FontLoadStatus,
+    UnifiedFont,
+} from '../../model/types';
+
+/**
+ * Options for {@link createFontRowSizeResolver}.
+ *
+ * All getter functions are called on every resolver invocation. When called
+ * inside a Svelte `$derived.by` block, any reactive state read within them
+ * (e.g. `SvelteMap.get()`) is automatically tracked as a dependency.
+ */
+export interface FontRowSizeResolverOptions {
+    /**
+     * Returns the current fonts array. Index `i` corresponds to row `i`.
+     */
+    getFonts: () => UnifiedFont[];
+    /**
+     * Returns the active font weight (e.g. 400).
+     */
+    getWeight: () => number;
+    /**
+     * Returns the preview text string.
+     */
+    getPreviewText: () => string;
+    /**
+     * Returns the scroll container's inner width in pixels. Returns 0 before mount.
+     */
+    getContainerWidth: () => number;
+    /**
+     * Returns the font size in pixels (e.g. `controlManager.renderedSize`).
+     */
+    getFontSizePx: () => number;
+    /**
+     * Returns the computed line height in pixels.
+     * Typically `controlManager.height * controlManager.renderedSize`.
+     */
+    getLineHeightPx: () => number;
+    /**
+     * Returns the font load status for a given font key (`'{id}@{weight}'` or `'{id}@vf'`).
+     *
+     * In production: `(key) => fontLifecycleManager.statuses.get(key)`.
+     * Injected for testability — avoids a module-level singleton dependency in tests.
+     * The call to `.get()` on a `SvelteMap` must happen inside a `$derived.by` context
+     * for reactivity to work. This is satisfied when `itemHeight` is called by
+     * `createVirtualizer`'s `estimateSize`.
+     */
+    getStatus: (fontKey: string) => FontLoadStatus | undefined;
+    /**
+     * Total horizontal padding of the text content area in pixels.
+     * Use the smallest breakpoint value (mobile `p-4` = 32px) to guarantee
+     * the content width is never over-estimated, keeping the height estimate safe.
+     */
+    contentHorizontalPadding: number;
+    /**
+     * Fixed height in pixels of chrome that is not text content (header bar, etc.).
+     */
+    chromeHeight: number;
+    /**
+     * Height in pixels to return when the font is not loaded or container width is 0.
+     */
+    fallbackHeight: number;
+}
+
+/**
+ * Creates a row-height resolver for `FontSampler` rows in `VirtualList`.
+ *
+ * The returned function is suitable as the `itemHeight` prop of `VirtualList`.
+ * Pass it from the widget layer (`SampleList`) so that typography values from
+ * `controlManager` are injected as getter functions rather than imported directly,
+ * keeping `$entities/Font` free of `$features` dependencies.
+ *
+ * **Reactivity:** When the returned function reads `getStatus()` inside a
+ * `$derived.by` block (as `estimateSize` does in `createVirtualizer`), any
+ * `SvelteMap.get()` call within `getStatus` registers a Svelte 5 dependency.
+ * When a font's status changes to `'loaded'`, `offsets` recomputes automatically —
+ * no DOM snap occurs.
+ *
+ * **Caching:** A `Map` keyed by `fontCssString|text|contentWidth|lineHeightPx`
+ * prevents redundant `pretext.layout()` calls. The cache is invalidated
+ * naturally because a change in any input produces a different cache key.
+ *
+ * @param options - Configuration and getter functions (all injected for testability).
+ * @returns A function `(rowIndex: number) => number` for use as `VirtualList.itemHeight`.
+ */
+export function createFontRowSizeResolver(options: FontRowSizeResolverOptions): (rowIndex: number) => number {
+    // Key: `${fontCssString}|${text}|${contentWidth}|${lineHeightPx}`
+    const cache = new Map<string, number>();
+
+    return function resolveRowHeight(rowIndex: number): number {
+        const fonts = options.getFonts();
+        const font = fonts[rowIndex];
+        if (!font) {
+            return options.fallbackHeight;
+        }
+
+        const containerWidth = options.getContainerWidth();
+        const previewText = options.getPreviewText();
+
+        if (containerWidth <= 0 || !previewText) {
+            return options.fallbackHeight;
+        }
+
+        const weight = options.getWeight();
+        // generateFontKey: '{id}@{weight}' for static fonts, '{id}@vf' for variable fonts.
+        const fontKey = generateFontKey({ id: font.id, weight, isVariable: font.features?.isVariable });
+
+        // Reading via getStatus() allows the caller to pass fontLifecycleManager.statuses.get(),
+        // which creates a Svelte 5 reactive dependency when called inside $derived.by.
+        const status = options.getStatus(fontKey);
+        if (status !== 'loaded') {
+            return options.fallbackHeight;
+        }
+
+        const fontSizePx = options.getFontSizePx();
+        const lineHeightPx = options.getLineHeightPx();
+        const contentWidth = containerWidth - options.contentHorizontalPadding;
+        const fontCssString = `${weight} ${fontSizePx}px "${font.name}"`;
+
+        const cacheKey = `${fontCssString}|${previewText}|${contentWidth}|${lineHeightPx}`;
+        const cached = cache.get(cacheKey);
+        if (cached !== undefined) {
+            return cached;
+        }
+
+        // Pretext docs recommend `layout()` (not `layoutWithLines`) for the
+        // resize hot path — pure arithmetic on cached segment widths, no canvas
+        // calls, no string allocations.
+        const prepared = prepare(previewText, fontCssString);
+        const { height: totalHeight } = layout(prepared, contentWidth, lineHeightPx);
+        const result = totalHeight + options.chromeHeight;
+        cache.set(cacheKey, result);
+        return result;
+    };
+}

src/entities/Font/model/const/const.ts

@@ -29,3 +29,9 @@ export const DEFAULT_LETTER_SPACING = 0;
 export const MIN_LETTER_SPACING = -0.1;
 export const MAX_LETTER_SPACING = 0.5;
 export const LETTER_SPACING_STEP = 0.01;
+
+/**
+ * Index value for items not yet loaded in a virtualized list.
+ * Treated as being at the very bottom of the infinite scroll.
+ */
+export const VIRTUAL_INDEX_NOT_LOADED = Infinity;

src/entities/Font/model/index.ts

@@ -1,44 +1,51 @@
+export {
+    DEFAULT_FONT_SIZE,
+    DEFAULT_FONT_WEIGHT,
+    DEFAULT_LETTER_SPACING,
+    DEFAULT_LINE_HEIGHT,
+    FONT_SIZE_STEP,
+    FONT_WEIGHT_STEP,
+    LETTER_SPACING_STEP,
+    LINE_HEIGHT_STEP,
+    MAX_FONT_SIZE,
+    MAX_FONT_WEIGHT,
+    MAX_LETTER_SPACING,
+    MAX_LINE_HEIGHT,
+    MIN_FONT_SIZE,
+    MIN_FONT_WEIGHT,
+    MIN_LETTER_SPACING,
+    MIN_LINE_HEIGHT,
+    VIRTUAL_INDEX_NOT_LOADED,
+} from './const/const';
+
+// Stores (lazy accessors + classes)
+export {
+    __resetFontLifecycleManager,
+    FontLifecycleManager,
+    FontsByIdsStore,
+    getFontCatalog,
+    getFontLifecycleManager,
+} from './store';
+export type { FontCatalogStore } from './store';
+
 export type {
-    // Domain types
+    FilterGroup,
+    FilterType,
     FontCategory,
     FontCollectionFilters,
     FontCollectionSort,
-    // Store types
     FontCollectionState,
     FontFeatures,
-    FontFiles,
-    FontItem,
+    FontFilters,
+    FontLoadRequestConfig,
+    FontLoadStatus,
     FontMetadata,
     FontProvider,
-    // Fontshare API types
-    FontshareApiModel,
-    FontshareAxis,
-    FontshareDesigner,
-    FontshareFeature,
-    FontshareFont,
-    FontshareLink,
-    FontsharePublisher,
-    FontshareStyle,
-    FontshareStyleProperties,
-    FontshareTag,
-    FontshareWeight,
     FontStyleUrls,
     FontSubset,
     FontVariant,
     FontWeight,
     FontWeightItalic,
-    // Google Fonts API types
-    GoogleFontsApiModel,
-    // Normalization types
     UnifiedFont,
     UnifiedFontVariant,
 } from './types';
-
-export {
-    appliedFontsManager,
-    createUnifiedFontStore,
-    type FontConfigRequest,
-    selectedFontsStore,
-    type UnifiedFontStore,
-    unifiedFontStore,
-} from './store';

src/entities/Font/model/store/appliedFontsStore/appliedFontsStore.svelte.ts

@@ -1,186 +0,0 @@
-import { SvelteMap } from 'svelte/reactivity';
-
-export type FontStatus = 'loading' | 'loaded' | 'error';
-
-export interface FontConfigRequest {
-    /**
-     * Font id
-     */
-    id: string;
-    /**
-     * Real font name (e.g. "Lato")
-     */
-    name: string;
-    /**
-     * The .ttf URL
-     */
-    url: string;
-    /**
-     * Font weight
-     */
-    weight: number;
-    /**
-     * Flag of the variable weight
-     */
-    isVariable?: boolean;
-}
-
-/**
- * Manager that handles loading of fonts.
- * Logic:
- * - Variable fonts: Loaded once per id (covers all weights).
- * - Static fonts: Loaded per id + weight combination.
- */
-class AppliedFontsManager {
-    #usageTracker = new Map<string, number>();
-    #idToBatch = new Map<string, string>();
-    // Changed to HTMLStyleElement
-    #batchElements = new Map<string, HTMLStyleElement>();
-
-    #queue = new Map<string, FontConfigRequest>(); // Track config in queue
-    #timeoutId: ReturnType<typeof setTimeout> | null = null;
-
-    #PURGE_INTERVAL = 60000;
-    #TTL = 5 * 60 * 1000;
-    #CHUNK_SIZE = 5; // Can be larger since we're just injecting strings
-
-    statuses = new SvelteMap<string, FontStatus>();
-
-    constructor() {
-        if (typeof window !== 'undefined') {
-            setInterval(() => this.#purgeUnused(), this.#PURGE_INTERVAL);
-        }
-    }
-
-    #getFontKey(config: FontConfigRequest): string {
-        if (config.isVariable) {
-            // For variable fonts, the ID is unique enough.
-            // Loading "Roboto" once covers "Roboto 400" and "Roboto 700"
-            return `${config.id.toLowerCase()}@vf`;
-        }
-        // For static fonts, we still need weight separation
-        return `${config.id.toLowerCase()}@${config.weight}`;
-    }
-
-    touch(configs: FontConfigRequest[]) {
-        const now = Date.now();
-        configs.forEach(config => {
-            // Pass the whole config to get key
-            const key = this.#getFontKey(config);
-
-            this.#usageTracker.set(key, now);
-
-            // If it's already loaded, we don't need to do anything
-            if (this.statuses.get(key) === 'loaded') return;
-
-            if (!this.#idToBatch.has(key) && !this.#queue.has(key)) {
-                this.#queue.set(key, config);
-
-                if (this.#timeoutId) clearTimeout(this.#timeoutId);
-                this.#timeoutId = setTimeout(() => this.#processQueue(), 50);
-            }
-        });
-    }
-
-    getFontStatus(id: string, weight: number, isVariable: boolean = false) {
-        // Construct a temp config to generate key
-        const key = this.#getFontKey({ id, weight, name: '', url: '', isVariable });
-        return this.statuses.get(key);
-    }
-
-    #processQueue() {
-        const entries = Array.from(this.#queue.entries());
-        if (entries.length === 0) return;
-
-        for (let i = 0; i < entries.length; i += this.#CHUNK_SIZE) {
-            this.#createBatch(entries.slice(i, i + this.#CHUNK_SIZE));
-        }
-
-        this.#queue.clear();
-        this.#timeoutId = null;
-    }
-
-    #createBatch(batchEntries: [string, FontConfigRequest][]) {
-        if (typeof document === 'undefined') return;
-
-        const batchId = crypto.randomUUID();
-        let cssRules = '';
-
-        batchEntries.forEach(([key, config]) => {
-            this.statuses.set(key, 'loading');
-            this.#idToBatch.set(key, batchId);
-
-            // If variable, allow the full weight range.
-            // If static, lock it to the specific weight.
-            const weightRule = config.isVariable
-                ? '100 900' // Variable range (standard coverage)
-                : config.weight;
-            const fontFormat = config.isVariable ? 'truetype-variations' : 'truetype';
-
-            cssRules += `
-                    @font-face {
-                        font-family: '${config.name}';
-                        src: url('${config.url}') format('${fontFormat}');
-                        font-weight: ${weightRule};
-                        font-style: normal;
-                        font-display: swap;
-                    }
-                `;
-        });
-
-        const style = document.createElement('style');
-        style.dataset.batchId = batchId;
-        style.innerHTML = cssRules;
-        document.head.appendChild(style);
-        this.#batchElements.set(batchId, style);
-
-        // Use the requested weight for verification, even if the rule covers a range
-        batchEntries.forEach(([key, config]) => {
-            document.fonts.load(`${config.weight} 1em "${config.name}"`)
-                .then(loaded => {
-                    this.statuses.set(key, loaded.length > 0 ? 'loaded' : 'error');
-                })
-                .catch(() => this.statuses.set(key, 'error'));
-        });
-    }
-    #purgeUnused() {
-        const now = Date.now();
-        const batchesToRemove = new Set<string>();
-        const keysToRemove: string[] = [];
-
-        for (const [key, lastUsed] of this.#usageTracker.entries()) {
-            if (now - lastUsed > this.#TTL) {
-                const batchId = this.#idToBatch.get(key);
-                if (batchId) {
-                    // Check if EVERY font in this batch is expired
-                    const batchKeys = Array.from(this.#idToBatch.entries())
-                        .filter(([_, bId]) => bId === batchId)
-                        .map(([k]) => k);
-
-                    const canDeleteBatch = batchKeys.every(k => {
-                        const lastK = this.#usageTracker.get(k);
-                        return lastK && (now - lastK > this.#TTL);
-                    });
-
-                    if (canDeleteBatch) {
-                        batchesToRemove.add(batchId);
-                        keysToRemove.push(...batchKeys);
-                    }
-                }
-            }
-        }
-
-        batchesToRemove.forEach(id => {
-            this.#batchElements.get(id)?.remove();
-            this.#batchElements.delete(id);
-        });
-
-        keysToRemove.forEach(k => {
-            this.#idToBatch.delete(k);
-            this.#usageTracker.delete(k);
-            this.statuses.delete(k);
-        });
-    }
-}
-
-export const appliedFontsManager = new AppliedFontsManager();

src/entities/Font/model/store/baseFontStore.svelte.ts

@@ -1,158 +0,0 @@
-import { queryClient } from '$shared/api/queryClient';
-import {
-    type QueryKey,
-    QueryObserver,
-    type QueryObserverOptions,
-    type QueryObserverResult,
-} from '@tanstack/query-core';
-import type { UnifiedFont } from '../types';
-
-/** */
-export abstract class BaseFontStore<TParams extends Record<string, any>> {
-    cleanup: () => void;
-
-    #bindings = $state<(() => Partial<TParams>)[]>([]);
-    #internalParams = $state<TParams>({} as TParams);
-
-    params = $derived.by(() => {
-        let merged = { ...this.#internalParams };
-
-        // Loop through every "Cable" plugged into the store
-        // Loop through every "Cable" plugged into the store
-        for (const getter of this.#bindings) {
-            const bindingResult = getter();
-            merged = { ...merged, ...bindingResult };
-        }
-
-        return merged as TParams;
-    });
-
-    protected result = $state<QueryObserverResult<UnifiedFont[], Error>>({} as any);
-    protected observer: QueryObserver<UnifiedFont[], Error>;
-    protected qc = queryClient;
-
-    constructor(initialParams: TParams) {
-        this.#internalParams = initialParams;
-
-        this.observer = new QueryObserver(this.qc, this.getOptions());
-
-        // Sync TanStack -> Svelte State
-        this.observer.subscribe(r => {
-            this.result = r;
-        });
-
-        // Sync Svelte State -> TanStack Options
-        this.cleanup = $effect.root(() => {
-            $effect(() => {
-                this.observer.setOptions(this.getOptions());
-            });
-        });
-    }
-
-    /**
-     * Mandatory: Child must define how to fetch data and what the key is.
-     */
-    protected abstract getQueryKey(params: TParams): QueryKey;
-    protected abstract fetchFn(params: TParams): Promise<UnifiedFont[]>;
-
-    protected getOptions(params = this.params): QueryObserverOptions<UnifiedFont[], Error> {
-        return {
-            queryKey: this.getQueryKey(params),
-            queryFn: () => this.fetchFn(params),
-            staleTime: 5 * 60 * 1000,
-            gcTime: 10 * 60 * 1000,
-        };
-    }
-
-    // --- Common Getters ---
-    get fonts() {
-        return this.result.data ?? [];
-    }
-    get isLoading() {
-        return this.result.isLoading;
-    }
-    get isFetching() {
-        return this.result.isFetching;
-    }
-    get isError() {
-        return this.result.isError;
-    }
-    get isEmpty() {
-        return !this.isLoading && this.fonts.length === 0;
-    }
-
-    // --- Common Actions ---
-
-    addBinding(getter: () => Partial<TParams>) {
-        this.#bindings.push(getter);
-
-        return () => {
-            this.#bindings = this.#bindings.filter(b => b !== getter);
-        };
-    }
-
-    setParams(newParams: Partial<TParams>) {
-        this.#internalParams = { ...this.params, ...newParams };
-    }
-    /**
-     * Invalidate cache and refetch
-     */
-    invalidate() {
-        this.qc.invalidateQueries({ queryKey: this.getQueryKey(this.params) });
-    }
-
-    destroy() {
-        this.cleanup();
-    }
-
-    /**
-     * Manually refetch
-     */
-    async refetch() {
-        await this.observer.refetch();
-    }
-
-    /**
-     * Prefetch with different params (for hover states, pagination, etc.)
-     */
-    async prefetch(params: TParams) {
-        await this.qc.prefetchQuery(this.getOptions(params));
-    }
-
-    /**
-     * Cancel ongoing queries
-     */
-    cancel() {
-        this.qc.cancelQueries({
-            queryKey: this.getQueryKey(this.params),
-        });
-    }
-
-    /**
-     * Clear cache for current params
-     */
-    clearCache() {
-        this.qc.removeQueries({
-            queryKey: this.getQueryKey(this.params),
-        });
-    }
-
-    /**
-     * Get cached data without triggering fetch
-     */
-    getCachedData() {
-        return this.qc.getQueryData<UnifiedFont[]>(
-            this.getQueryKey(this.params),
-        );
-    }
-
-    /**
-     * Set data manually (optimistic updates)
-     */
-    setQueryData(updater: (old: UnifiedFont[] | undefined) => UnifiedFont[]) {
-        this.qc.setQueryData(
-            this.getQueryKey(this.params),
-            updater,
-        );
-    }
-}

src/entities/Font/model/store/fontCatalogStore/fontCatalogStore.svelte.spec.ts

@@ -0,0 +1,641 @@
+import {
+    generateMixedCategoryFonts,
+    generateMockFonts,
+} from '$entities/Font/testing';
+import { flushSync } from 'svelte';
+import {
+    afterEach,
+    beforeEach,
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import {
+    FontNetworkError,
+    FontResponseError,
+} from '../../../lib/errors/errors';
+import type { UnifiedFont } from '../../types';
+import { FontCatalogStore } from './fontCatalogStore.svelte';
+
+vi.mock('$shared/api/queryClient', async importOriginal => {
+    /**
+     * Import QueryClient inside the factory rather than referencing the top-level binding.
+     * A hoisted vi.mock factory that touches a module-level import can hit that import
+     * before it is initialized (ReferenceError) when the import sits in a circular/eager
+     * barrel chain — which it now does via $shared/lib → BaseQueryStore → query-core.
+     */
+    const { QueryClient } = await import('@tanstack/query-core');
+    const actual = await importOriginal<typeof import('$shared/api/queryClient')>();
+    const mockClient = new QueryClient({
+        defaultOptions: { queries: { retry: 0, gcTime: 0 } },
+    });
+    return {
+        ...actual,
+        getQueryClient: () => mockClient,
+    };
+});
+vi.mock('../../../api', () => ({ fetchProxyFonts: vi.fn() }));
+
+import { getQueryClient } from '$shared/api/queryClient';
+import { fetchProxyFonts } from '../../../api';
+
+const queryClient = getQueryClient();
+
+const fetch = fetchProxyFonts as ReturnType<typeof vi.fn>;
+
+type FontPage = { fonts: UnifiedFont[]; total: number; limit: number; offset: number };
+
+const makeResponse = (
+    fonts: UnifiedFont[],
+    meta: { total?: number; limit?: number; offset?: number } = {},
+): FontPage => ({
+    fonts,
+    total: meta.total ?? fonts.length,
+    limit: meta.limit ?? 10,
+    offset: meta.offset ?? 0,
+});
+
+function makeStore(params = {}) {
+    return new FontCatalogStore({ limit: 10, ...params });
+}
+
+async function fetchedStore(params = {}, fonts = generateMockFonts(5), meta: Parameters<typeof makeResponse>[1] = {}) {
+    fetch.mockResolvedValue(makeResponse(fonts, meta));
+    const store = makeStore(params);
+    await store.refetch();
+    flushSync();
+    return store;
+}
+
+describe('FontCatalogStore', () => {
+    afterEach(() => {
+        queryClient.clear();
+        vi.resetAllMocks();
+    });
+
+    describe('construction', () => {
+        it('stores initial params', () => {
+            const store = makeStore({ limit: 20 });
+            expect(store.params.limit).toBe(20);
+            store.destroy();
+        });
+
+        it('defaults limit to 50 when not provided', () => {
+            const store = new FontCatalogStore();
+            expect(store.params.limit).toBe(50);
+            store.destroy();
+        });
+
+        it('starts with empty fonts', () => {
+            const store = makeStore();
+            expect(store.fonts).toEqual([]);
+            store.destroy();
+        });
+
+        it('starts with isEmpty false — observer is gated until setParams enables it', () => {
+            // The observer is disabled on construction (no auto-fetch) — see
+            // `#enabled` in the store. isEmpty must still be false so the UI
+            // doesn't flash "no results" before bindings configures the query.
+            const store = makeStore();
+            expect(store.isEmpty).toBe(false);
+            store.destroy();
+        });
+    });
+
+    describe('state after fetch', () => {
+        it('exposes loaded fonts', async () => {
+            const store = await fetchedStore({}, generateMockFonts(7));
+            expect(store.fonts).toHaveLength(7);
+            store.destroy();
+        });
+
+        it('isEmpty is false when fonts are present', async () => {
+            const store = await fetchedStore();
+            expect(store.isEmpty).toBe(false);
+            store.destroy();
+        });
+
+        it('isLoading is false after fetch', async () => {
+            const store = await fetchedStore();
+            expect(store.isLoading).toBe(false);
+            store.destroy();
+        });
+
+        it('isFetching is false after fetch', async () => {
+            const store = await fetchedStore();
+            expect(store.isFetching).toBe(false);
+            store.destroy();
+        });
+
+        it('isError is false on success', async () => {
+            const store = await fetchedStore();
+            expect(store.isError).toBe(false);
+            store.destroy();
+        });
+
+        it('error is null on success', async () => {
+            const store = await fetchedStore();
+            expect(store.error).toBeNull();
+            store.destroy();
+        });
+    });
+
+    describe('error states', () => {
+        it('isError is false before any fetch', () => {
+            const store = makeStore();
+            expect(store.isError).toBe(false);
+            store.destroy();
+        });
+
+        it('wraps network failures in FontNetworkError', async () => {
+            fetch.mockRejectedValue(new Error('network down'));
+            const store = makeStore();
+            await store.refetch().catch(() => {});
+            flushSync();
+            expect(store.error).toBeInstanceOf(FontNetworkError);
+            expect(store.isError).toBe(true);
+            store.destroy();
+        });
+
+        it('exposes FontResponseError for falsy response', async () => {
+            const store = makeStore();
+            fetch.mockResolvedValue(null);
+            await store.refetch().catch(() => {});
+            flushSync();
+            expect(store.error).toBeInstanceOf(FontResponseError);
+            expect((store.error as FontResponseError).field).toBe('response');
+            store.destroy();
+        });
+
+        it('exposes FontResponseError for missing fonts field', async () => {
+            fetch.mockResolvedValue({ total: 0, limit: 10, offset: 0 });
+            const store = makeStore();
+            await store.refetch().catch(() => {});
+            flushSync();
+            expect(store.error).toBeInstanceOf(FontResponseError);
+            expect((store.error as FontResponseError).field).toBe('response.fonts');
+            store.destroy();
+        });
+
+        it('exposes FontResponseError for non-array fonts', async () => {
+            fetch.mockResolvedValue({ fonts: 'bad', total: 0, limit: 10, offset: 0 });
+            const store = makeStore();
+            await store.refetch().catch(() => {});
+            flushSync();
+            expect(store.error).toBeInstanceOf(FontResponseError);
+            expect((store.error as FontResponseError).received).toBe('bad');
+            store.destroy();
+        });
+    });
+
+    describe('font accumulation', () => {
+        it('replaces fonts when refetching the first page', async () => {
+            const store = makeStore();
+            fetch.mockResolvedValue(makeResponse(generateMockFonts(3)));
+            await store.refetch();
+            flushSync();
+
+            const second = generateMockFonts(2);
+            fetch.mockResolvedValue(makeResponse(second));
+            await store.refetch();
+            flushSync();
+
+            // refetch at offset=0 re-fetches all pages; only one page loaded → new data replaces old
+            expect(store.fonts).toHaveLength(2);
+            expect(store.fonts[0].id).toBe(second[0].id);
+            store.destroy();
+        });
+
+        it('appends fonts after nextPage', async () => {
+            const page1 = generateMockFonts(3);
+            const store = await fetchedStore({ limit: 3 }, page1, { total: 6, limit: 3, offset: 0 });
+            const page2 = generateMockFonts(3).map((f, i) => ({ ...f, id: `p2-${i}` }));
+            fetch.mockResolvedValue(makeResponse(page2, { total: 6, limit: 3, offset: 3 }));
+            await store.nextPage();
+            flushSync();
+
+            expect(store.fonts).toHaveLength(6);
+            expect(store.fonts.slice(0, 3).map(f => f.id)).toEqual(page1.map(f => f.id));
+            expect(store.fonts.slice(3).map(f => f.id)).toEqual(page2.map(f => f.id));
+            store.destroy();
+        });
+    });
+
+    describe('pagination state', () => {
+        it('returns zero-value defaults before any fetch', () => {
+            const store = makeStore();
+            expect(store.pagination).toMatchObject({ total: 0, hasMore: false, page: 1, totalPages: 0 });
+            store.destroy();
+        });
+
+        it('reflects response metadata after fetch', async () => {
+            const store = await fetchedStore({}, generateMockFonts(10), { total: 30, limit: 10, offset: 0 });
+            expect(store.pagination.total).toBe(30);
+            expect(store.pagination.hasMore).toBe(true);
+            expect(store.pagination.page).toBe(1);
+            expect(store.pagination.totalPages).toBe(3);
+            store.destroy();
+        });
+
+        it('hasMore is false on the last page', async () => {
+            const store = await fetchedStore({}, generateMockFonts(10), { total: 10, limit: 10, offset: 0 });
+            expect(store.pagination.hasMore).toBe(false);
+            store.destroy();
+        });
+
+        it('page count increments after nextPage', async () => {
+            const store = await fetchedStore({ limit: 10 }, generateMockFonts(10), { total: 30, limit: 10, offset: 0 });
+            expect(store.pagination.page).toBe(1);
+
+            fetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 30, limit: 10, offset: 10 }));
+            await store.nextPage();
+            flushSync();
+            expect(store.pagination.page).toBe(2);
+
+            store.destroy();
+        });
+    });
+
+    describe('setParams', () => {
+        it('merges updates into existing params', () => {
+            const store = makeStore({ limit: 10 });
+            store.setParams({ limit: 20 });
+            expect(store.params.limit).toBe(20);
+            store.destroy();
+        });
+
+        it('retains unmodified params', () => {
+            const store = makeStore({ limit: 10 });
+            store.setCategories(['serif']);
+            store.setParams({ limit: 25 });
+            expect(store.params.categories).toEqual(['serif']);
+            store.destroy();
+        });
+    });
+
+    describe('filter change resets', () => {
+        it('clears accumulated fonts when a filter changes', async () => {
+            const store = await fetchedStore({}, generateMockFonts(5));
+            store.setSearch('roboto');
+            flushSync();
+            // TQ switches to a new queryKey → data.pages reset → fonts = []
+            expect(store.fonts).toHaveLength(0);
+            store.destroy();
+        });
+
+        it('isEmpty is false immediately after filter change — fetch is in progress', async () => {
+            const store = await fetchedStore({}, generateMockFonts(5));
+            // Hang the next fetch so we can observe the transitioning state
+            fetch.mockReturnValue(new Promise(() => {}));
+            store.setSearch('roboto');
+            flushSync();
+            // fonts = [] AND isFetching = true → isEmpty must be false (no "no results" flash)
+            expect(store.isEmpty).toBe(false);
+            store.destroy();
+        });
+
+        it('does NOT reset fonts when the same filter value is set again', async () => {
+            const store = await fetchedStore({}, generateMockFonts(5));
+            store.setCategories(['serif']);
+            flushSync();
+            // First change: clears fonts (expected)
+            store.setCategories(['serif']); // same value — same queryKey — TQ keeps data.pages
+            flushSync();
+            // Because queryKey hasn't changed, TQ returns cached data — fonts restored from cache
+            // (actual font count depends on cache; key assertion is no extra reset)
+            expect(store.isError).toBe(false);
+            store.destroy();
+        });
+    });
+
+    describe('staleTime in buildOptions', () => {
+        it('is 5 minutes with no active filters', () => {
+            const store = makeStore();
+            expect((store as any).buildOptions().staleTime).toBe(5 * 60 * 1000);
+            store.destroy();
+        });
+
+        it('is 0 when a search query is active', () => {
+            const store = makeStore();
+            store.setSearch('roboto');
+            expect((store as any).buildOptions().staleTime).toBe(0);
+            store.destroy();
+        });
+
+        it('is 0 when a category filter is active', () => {
+            const store = makeStore();
+            store.setCategories(['serif']);
+            expect((store as any).buildOptions().staleTime).toBe(0);
+            store.destroy();
+        });
+
+        it('gcTime is 10 minutes always', () => {
+            const store = makeStore();
+            expect((store as any).buildOptions().gcTime).toBe(10 * 60 * 1000);
+            store.destroy();
+        });
+    });
+
+    describe('buildQueryKey', () => {
+        it('omits empty-string params', () => {
+            const store = makeStore();
+            store.setSearch('');
+            const [root, normalized] = (store as any).buildQueryKey(store.params);
+            expect(root).toBe('fonts');
+            expect(normalized).not.toHaveProperty('q');
+            store.destroy();
+        });
+
+        it('omits empty-array params', () => {
+            const store = makeStore();
+            store.setProviders([]);
+            const [, normalized] = (store as any).buildQueryKey(store.params);
+            expect(normalized).not.toHaveProperty('providers');
+            store.destroy();
+        });
+
+        it('includes non-empty filter values', () => {
+            const store = makeStore();
+            store.setCategories(['serif']);
+            const [, normalized] = (store as any).buildQueryKey(store.params);
+            expect(normalized).toHaveProperty('categories', ['serif']);
+            store.destroy();
+        });
+
+        it('does not include offset (offset is the TQ page param, not a query key component)', () => {
+            const store = makeStore();
+            const [, normalized] = (store as any).buildQueryKey(store.params);
+            expect(normalized).not.toHaveProperty('offset');
+            store.destroy();
+        });
+    });
+
+    describe('destroy', () => {
+        it('does not throw', () => {
+            const store = makeStore();
+            expect(() => store.destroy()).not.toThrow();
+        });
+
+        it('is idempotent', () => {
+            const store = makeStore();
+            store.destroy();
+            expect(() => store.destroy()).not.toThrow();
+        });
+    });
+
+    describe('refetch', () => {
+        it('triggers a fetch', async () => {
+            const store = makeStore();
+            fetch.mockResolvedValue(makeResponse(generateMockFonts(3)));
+            await store.refetch();
+            expect(fetch).toHaveBeenCalled();
+            store.destroy();
+        });
+
+        it('uses params current at call time', async () => {
+            const store = makeStore({ limit: 10 });
+            store.setParams({ limit: 20 });
+            fetch.mockResolvedValue(makeResponse(generateMockFonts(20)));
+            await store.refetch();
+            expect(fetch).toHaveBeenCalledWith(expect.objectContaining({ limit: 20 }));
+            store.destroy();
+        });
+    });
+
+    describe('nextPage', () => {
+        let store: FontCatalogStore;
+
+        beforeEach(async () => {
+            fetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 30, limit: 10, offset: 0 }));
+            store = new FontCatalogStore({ limit: 10 });
+            await store.refetch();
+            flushSync();
+        });
+
+        afterEach(() => {
+            store.destroy();
+        });
+
+        it('fetches the next page and appends fonts', async () => {
+            fetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 30, limit: 10, offset: 10 }));
+            await store.nextPage();
+            flushSync();
+            expect(store.fonts).toHaveLength(20);
+            expect(store.pagination.offset).toBe(10);
+        });
+
+        it('is a no-op when hasMore is false', async () => {
+            // Set up a store where all fonts fit in one page (hasMore = false)
+            queryClient.clear();
+            fetch.mockResolvedValue(makeResponse(generateMockFonts(10), { total: 10, limit: 10, offset: 0 }));
+            store = new FontCatalogStore({ limit: 10 });
+            await store.refetch();
+            flushSync();
+
+            expect(store.pagination.hasMore).toBe(false);
+            await store.nextPage(); // should not trigger another fetch
+            expect(store.fonts).toHaveLength(10);
+        });
+    });
+
+    describe('prevPage and goToPage', () => {
+        it('prevPage is a no-op — infinite scroll does not support backward navigation', async () => {
+            const store = await fetchedStore({}, generateMockFonts(5));
+            store.prevPage();
+            expect(store.fonts).toHaveLength(5); // unchanged
+            store.destroy();
+        });
+
+        it('goToPage is a no-op — infinite scroll does not support arbitrary page jumps', async () => {
+            const store = await fetchedStore({}, generateMockFonts(5));
+            store.goToPage(3);
+            expect(store.fonts).toHaveLength(5); // unchanged
+            store.destroy();
+        });
+    });
+
+    describe('prefetch', () => {
+        it('triggers a fetch for the provided params', async () => {
+            const store = makeStore();
+            fetch.mockResolvedValue(makeResponse(generateMockFonts(5)));
+            await store.prefetch({ limit: 5 });
+            expect(fetch).toHaveBeenCalledWith(expect.objectContaining({ limit: 5, offset: 0 }));
+            store.destroy();
+        });
+    });
+
+    describe('getCachedData / setQueryData', () => {
+        it('getCachedData returns undefined before any fetch', () => {
+            queryClient.clear();
+            const store = new FontCatalogStore({ limit: 10 });
+            expect(store.getCachedData()).toBeUndefined();
+            store.destroy();
+        });
+
+        it('getCachedData returns flattened fonts after fetch', async () => {
+            const store = await fetchedStore();
+            expect(store.getCachedData()).toHaveLength(5);
+            store.destroy();
+        });
+
+        it('setQueryData writes to cache', () => {
+            const store = makeStore();
+            const font = generateMockFonts(1)[0];
+            store.setQueryData(() => [font]);
+            expect(store.getCachedData()).toHaveLength(1);
+            store.destroy();
+        });
+
+        it('setQueryData updater receives existing flattened fonts', async () => {
+            const store = await fetchedStore();
+            const updater = vi.fn((old: UnifiedFont[] | undefined) => old ?? []);
+            store.setQueryData(updater);
+            expect(updater).toHaveBeenCalledWith(expect.any(Array));
+            store.destroy();
+        });
+    });
+
+    describe('invalidate', () => {
+        it('calls invalidateQueries', async () => {
+            const store = await fetchedStore();
+            const spy = vi.spyOn(queryClient, 'invalidateQueries');
+            store.invalidate();
+            expect(spy).toHaveBeenCalledOnce();
+            store.destroy();
+        });
+    });
+
+    describe('setLimit', () => {
+        it('updates the limit param', () => {
+            const store = makeStore({ limit: 10 });
+            store.setLimit(25);
+            expect(store.params.limit).toBe(25);
+            store.destroy();
+        });
+    });
+
+    describe('filter shortcut methods', () => {
+        let store: FontCatalogStore;
+
+        beforeEach(() => {
+            store = makeStore();
+        });
+        afterEach(() => {
+            store.destroy();
+        });
+
+        it('setProviders updates providers param', () => {
+            store.setProviders(['google']);
+            expect(store.params.providers).toEqual(['google']);
+        });
+
+        it('setCategories updates categories param', () => {
+            store.setCategories(['serif']);
+            expect(store.params.categories).toEqual(['serif']);
+        });
+
+        it('setSubsets updates subsets param', () => {
+            store.setSubsets(['cyrillic']);
+            expect(store.params.subsets).toEqual(['cyrillic']);
+        });
+
+        it('setSearch sets q param', () => {
+            store.setSearch('roboto');
+            expect(store.params.q).toBe('roboto');
+        });
+
+        it('setSearch with empty string clears q', () => {
+            store.setSearch('roboto');
+            store.setSearch('');
+            expect(store.params.q).toBeUndefined();
+        });
+
+        it('setSort updates sort param', () => {
+            store.setSort('popularity');
+            expect(store.params.sort).toBe('popularity');
+        });
+    });
+
+    describe('category getters', () => {
+        it('each getter returns only fonts of that category', async () => {
+            const fonts = generateMixedCategoryFonts(2); // 2 of each category = 10 total
+            fetch.mockResolvedValue(makeResponse(fonts, { total: fonts.length, limit: fonts.length }));
+            const store = makeStore({ limit: 50 });
+            await store.refetch();
+            flushSync();
+
+            expect(store.sansSerifFonts.every(f => f.category === 'sans-serif')).toBe(true);
+            expect(store.serifFonts.every(f => f.category === 'serif')).toBe(true);
+            expect(store.displayFonts.every(f => f.category === 'display')).toBe(true);
+            expect(store.handwritingFonts.every(f => f.category === 'handwriting')).toBe(true);
+            expect(store.monospaceFonts.every(f => f.category === 'monospace')).toBe(true);
+            expect(store.sansSerifFonts).toHaveLength(2);
+
+            store.destroy();
+        });
+    });
+
+    describe('fetchAllPagesTo', () => {
+        beforeEach(() => {
+            fetch.mockReset();
+            queryClient.clear();
+        });
+
+        it('fetches all missing pages in parallel up to targetIndex', async () => {
+            // First page already loaded (offset 0, limit 10, total 50)
+            const firstFonts = generateMockFonts(10);
+            fetch.mockResolvedValueOnce(makeResponse(firstFonts, { total: 50, limit: 10, offset: 0 }));
+            const store = makeStore();
+            await store.refetch();
+            flushSync();
+
+            expect(store.fonts).toHaveLength(10);
+
+            // Mock remaining pages
+            for (let offset = 10; offset < 50; offset += 10) {
+                fetch.mockResolvedValueOnce(
+                    makeResponse(generateMockFonts(10), { total: 50, limit: 10, offset }),
+                );
+            }
+
+            await store.fetchAllPagesTo(40);
+            flushSync();
+
+            expect(store.fonts).toHaveLength(50);
+        });
+
+        it('skips pages that fail and still merges successful ones', async () => {
+            const firstFonts = generateMockFonts(10);
+            fetch.mockResolvedValueOnce(makeResponse(firstFonts, { total: 30, limit: 10, offset: 0 }));
+            const store = makeStore();
+            await store.refetch();
+            flushSync();
+
+            // offset=10 fails, offset=20 succeeds
+            fetch.mockRejectedValueOnce(new Error('network error'));
+            fetch.mockResolvedValueOnce(
+                makeResponse(generateMockFonts(10), { total: 30, limit: 10, offset: 20 }),
+            );
+
+            await store.fetchAllPagesTo(25);
+            flushSync();
+
+            // Page at offset=20 merged, page at offset=10 missing — 20 total
+            expect(store.fonts).toHaveLength(20);
+        });
+
+        it('is a no-op when target is within already-loaded data', async () => {
+            const firstFonts = generateMockFonts(10);
+            fetch.mockResolvedValueOnce(makeResponse(firstFonts, { total: 50, limit: 10, offset: 0 }));
+            const store = makeStore();
+            await store.refetch();
+            flushSync();
+
+            const callsBefore = fetch.mock.calls.length;
+            await store.fetchAllPagesTo(5);
+
+            expect(fetch.mock.calls.length).toBe(callsBefore);
+        });
+    });
+});

src/entities/Font/model/store/fontCatalogStore/fontCatalogStore.svelte.ts

@@ -0,0 +1,495 @@
+import {
+    DEFAULT_QUERY_GC_TIME_MS,
+    DEFAULT_QUERY_STALE_TIME_MS,
+    getQueryClient,
+} from '$shared/api/queryClient';
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
+import {
+    type InfiniteData,
+    InfiniteQueryObserver,
+    type InfiniteQueryObserverResult,
+    type QueryFunctionContext,
+} from '@tanstack/query-core';
+import {
+    type ProxyFontsParams,
+    type ProxyFontsResponse,
+    fetchProxyFonts,
+} from '../../../api';
+import {
+    FontNetworkError,
+    FontResponseError,
+} from '../../../lib/errors/errors';
+import type { UnifiedFont } from '../../types';
+
+type PageParam = { offset: number };
+
+/**
+ * Filter params + limit — offset is managed by TQ as a page param, not a user param.
+ */
+type FontStoreParams = Omit<ProxyFontsParams, 'offset'>;
+
+type FontStoreResult = InfiniteQueryObserverResult<InfiniteData<ProxyFontsResponse, PageParam>, Error>;
+
+export class FontCatalogStore {
+    #params = $state<FontStoreParams>({ limit: 50 });
+    /**
+     * Gates the initial fetch. The observer starts disabled so the constructor
+     * cannot race ahead of the bindings module — which is the single source of
+     * truth for query params. The first setParams flips this on, producing a
+     * single fetch with the correctly merged queryKey.
+     */
+    #enabled = $state(false);
+    #result = $state<FontStoreResult>({} as FontStoreResult);
+    #observer: InfiniteQueryObserver<
+        ProxyFontsResponse,
+        Error,
+        InfiniteData<ProxyFontsResponse, PageParam>,
+        readonly unknown[],
+        PageParam
+    >;
+    #qc = getQueryClient();
+    #unsubscribe: () => void;
+
+    constructor(params: FontStoreParams = {}) {
+        this.#params = { limit: 50, ...params };
+        this.#observer = new InfiniteQueryObserver(this.#qc, this.buildOptions());
+        // Seed result synchronously; subscribe may not fire on disabled observers.
+        this.#result = this.#observer.getCurrentResult();
+        this.#unsubscribe = this.#observer.subscribe(r => {
+            this.#result = r;
+        });
+    }
+
+    /**
+     * Current filter and limit configuration
+     */
+    get params(): FontStoreParams {
+        return this.#params;
+    }
+    /**
+     * Flattened list of all fonts loaded across all pages (reactive)
+     */
+    get fonts(): UnifiedFont[] {
+        return this.#result.data?.pages.flatMap((p: ProxyFontsResponse) => p.fonts) ?? [];
+    }
+    /**
+     * True if the first page is currently being fetched
+     */
+    get isLoading(): boolean {
+        return this.#result.isLoading;
+    }
+    /**
+     * True if any background fetch is in progress (initial or pagination)
+     */
+    get isFetching(): boolean {
+        return this.#result.isFetching;
+    }
+    /**
+     * True if the last fetch attempt resulted in an error
+     */
+    get isError(): boolean {
+        return this.#result.isError;
+    }
+
+    /**
+     * Last caught error from the query observer
+     */
+    get error(): Error | null {
+        return this.#result.error ?? null;
+    }
+    /**
+     * True if no fonts were found for the current filter criteria.
+     * Always false until the observer has been enabled (via setParams) — otherwise
+     * the UI would briefly render "no results" on mount before bindings configures
+     * the query.
+     */
+    get isEmpty(): boolean {
+        return this.#enabled && !this.isLoading && !this.isFetching && this.fonts.length === 0;
+    }
+
+    /**
+     * Pagination metadata derived from the last loaded page
+     */
+    get pagination() {
+        const pages = this.#result.data?.pages;
+        const last = pages?.at(-1);
+        if (!last) {
+            return {
+                total: 0,
+                limit: this.#params.limit ?? 50,
+                offset: 0,
+                hasMore: false,
+                page: 1,
+                totalPages: 0,
+            };
+        }
+        return {
+            total: last.total,
+            limit: last.limit,
+            offset: last.offset,
+            hasMore: this.#result.hasNextPage,
+            page: pages!.length,
+            totalPages: Math.ceil(last.total / last.limit),
+        };
+    }
+
+    /**
+     * Cleans up subscriptions and destroys the observer
+     */
+    destroy() {
+        this.#unsubscribe();
+        this.#observer.destroy();
+    }
+
+    /**
+     * Merge new parameters into existing state and trigger a refetch.
+     * The first call also enables the observer (see `#enabled`).
+     */
+    setParams(updates: Partial<FontStoreParams>) {
+        this.#params = { ...this.#params, ...updates };
+        this.#enabled = true;
+        this.#observer.setOptions(this.buildOptions());
+    }
+    /**
+     * Forcefully invalidate and refetch the current query from the network
+     */
+    invalidate() {
+        this.#qc.invalidateQueries({ queryKey: this.buildQueryKey(this.#params) });
+    }
+
+    /**
+     * Manually trigger a query refetch
+     */
+    async refetch() {
+        await this.#observer.refetch();
+    }
+
+    /**
+     * Prime the cache with data for a specific parameter set
+     */
+    async prefetch(params: FontStoreParams) {
+        await this.#qc.prefetchInfiniteQuery(this.buildOptions(params));
+    }
+
+    /**
+     * Abort any active network requests for this store
+     */
+    cancel() {
+        this.#qc.cancelQueries({ queryKey: this.buildQueryKey(this.#params) });
+    }
+
+    /**
+     * Retrieve current font list from cache without triggering a fetch
+     */
+    getCachedData(): UnifiedFont[] | undefined {
+        const data = this.#qc.getQueryData<InfiniteData<ProxyFontsResponse, PageParam>>(
+            this.buildQueryKey(this.#params),
+        );
+        if (!data) {
+            return undefined;
+        }
+        return data.pages.flatMap(p => p.fonts);
+    }
+
+    /**
+     * Manually update the cached font data (useful for optimistic updates)
+     */
+    setQueryData(updater: (old: UnifiedFont[] | undefined) => UnifiedFont[]) {
+        const key = this.buildQueryKey(this.#params);
+        this.#qc.setQueryData<InfiniteData<ProxyFontsResponse, PageParam>>(
+            key,
+            old => {
+                const flatFonts = old?.pages.flatMap(p => p.fonts);
+                const newFonts = updater(flatFonts);
+                // Re-distribute the updated fonts back into the existing page structure
+                // Define the first page. If old data exists, we merge into the first page template.
+                const limit = typeof this.#params.limit === 'number' ? this.#params.limit : 50;
+                const template = old?.pages[0] ?? {
+                    total: newFonts.length,
+                    limit,
+                    offset: 0,
+                };
+
+                const updatedPage: ProxyFontsResponse = {
+                    ...template,
+                    fonts: newFonts,
+                    total: newFonts.length, // Synchronize total with the new font count
+                };
+
+                return {
+                    pages: [updatedPage],
+                    pageParams: [{ offset: 0 }],
+                };
+            },
+        );
+    }
+
+    /**
+     * Shortcut to update provider filters
+     */
+    setProviders(v: ProxyFontsParams['providers']) {
+        this.setParams({ providers: v });
+    }
+    /**
+     * Shortcut to update category filters
+     */
+    setCategories(v: ProxyFontsParams['categories']) {
+        this.setParams({ categories: v });
+    }
+    /**
+     * Shortcut to update subset filters
+     */
+    setSubsets(v: ProxyFontsParams['subsets']) {
+        this.setParams({ subsets: v });
+    }
+    /**
+     * Shortcut to update search query
+     */
+    setSearch(v: string) {
+        this.setParams({ q: v || undefined });
+    }
+    /**
+     * Shortcut to update sort order
+     */
+    setSort(v: ProxyFontsParams['sort']) {
+        this.setParams({ sort: v });
+    }
+
+    /**
+     * Fetch the next page of results if available
+     */
+    async nextPage(): Promise<void> {
+        await this.#observer.fetchNextPage();
+    }
+
+    #isCatchingUp = false;
+    #inFlightOffsets = new Set<number>();
+
+    /**
+     * Fetch all pages between the current loaded count and targetIndex in parallel.
+     * Pages are merged into the cache as they arrive (sorted by offset).
+     * Failed pages are silently skipped — normal scroll will re-fetch them on demand.
+     */
+    async fetchAllPagesTo(targetIndex: number): Promise<void> {
+        if (this.#isCatchingUp) {
+            return;
+        }
+
+        const pageSize = typeof this.#params.limit === 'number' ? this.#params.limit : 50;
+        const key = this.buildQueryKey(this.#params);
+        const existing = this.#qc.getQueryData<InfiniteData<ProxyFontsResponse, PageParam>>(key);
+
+        if (!existing) {
+            return;
+        }
+
+        const loadedOffsets = new Set(existing.pageParams.map(p => p.offset));
+
+        // Collect offsets for all missing and not-in-flight pages
+        const missingOffsets: number[] = [];
+        for (let offset = 0; offset <= targetIndex; offset += pageSize) {
+            if (!loadedOffsets.has(offset) && !this.#inFlightOffsets.has(offset)) {
+                missingOffsets.push(offset);
+            }
+        }
+
+        if (missingOffsets.length === 0) {
+            return;
+        }
+
+        this.#isCatchingUp = true;
+
+        // Sorted merge buffer — flush in offset order as pages arrive
+        const buffer = new Map<number, ProxyFontsResponse>();
+        const failed = new Set<number>();
+        let nextFlushOffset = (existing.pageParams.at(-1)?.offset ?? -pageSize) + pageSize;
+
+        const flush = () => {
+            while (buffer.has(nextFlushOffset) || failed.has(nextFlushOffset)) {
+                if (buffer.has(nextFlushOffset)) {
+                    this.#appendPageToCache(buffer.get(nextFlushOffset)!);
+                    buffer.delete(nextFlushOffset);
+                }
+                failed.delete(nextFlushOffset);
+                nextFlushOffset += pageSize;
+            }
+        };
+
+        try {
+            await Promise.allSettled(
+                missingOffsets.map(async offset => {
+                    this.#inFlightOffsets.add(offset);
+                    try {
+                        const page = await this.fetchPage({ ...this.#params, offset });
+                        buffer.set(offset, page);
+                    } catch {
+                        failed.add(offset);
+                    } finally {
+                        this.#inFlightOffsets.delete(offset);
+                    }
+                    flush();
+                }),
+            );
+        } finally {
+            this.#isCatchingUp = false;
+        }
+    }
+
+    /**
+     * Backward pagination (no-op: infinite scroll accumulates forward only)
+     */
+    prevPage(): void {}
+    /**
+     * Jump to specific page (no-op for infinite scroll)
+     */
+    goToPage(_page: number): void {}
+
+    /**
+     * Update the number of items fetched per page
+     */
+    setLimit(limit: number) {
+        this.setParams({ limit });
+    }
+
+    /**
+     * Derived list of sans-serif fonts in the current set
+     */
+    get sansSerifFonts() {
+        return this.fonts.filter(f => f.category === 'sans-serif');
+    }
+    /**
+     * Derived list of serif fonts in the current set
+     */
+    get serifFonts() {
+        return this.fonts.filter(f => f.category === 'serif');
+    }
+    /**
+     * Derived list of display fonts in the current set
+     */
+    get displayFonts() {
+        return this.fonts.filter(f => f.category === 'display');
+    }
+    /**
+     * Derived list of handwriting fonts in the current set
+     */
+    get handwritingFonts() {
+        return this.fonts.filter(f => f.category === 'handwriting');
+    }
+    /**
+     * Derived list of monospace fonts in the current set
+     */
+    get monospaceFonts() {
+        return this.fonts.filter(f => f.category === 'monospace');
+    }
+
+    /**
+     * Merge a single page into the InfiniteQuery cache in offset order.
+     * Called by fetchAllPagesTo as each parallel fetch resolves.
+     */
+    #appendPageToCache(page: ProxyFontsResponse): void {
+        const key = this.buildQueryKey(this.#params);
+        const existing = this.#qc.getQueryData<InfiniteData<ProxyFontsResponse, PageParam>>(key);
+        if (!existing) {
+            return;
+        }
+
+        // Guard against duplicates
+        const loadedOffsets = new Set(existing.pageParams.map(p => p.offset));
+        if (loadedOffsets.has(page.offset)) {
+            return;
+        }
+
+        const allPages = [...existing.pages, page].sort((a, b) => a.offset - b.offset);
+        const allParams = [...existing.pageParams, { offset: page.offset }].sort(
+            (a, b) => a.offset - b.offset,
+        );
+
+        this.#qc.setQueryData<InfiniteData<ProxyFontsResponse, PageParam>>(key, {
+            pages: allPages,
+            pageParams: allParams,
+        });
+    }
+
+    private buildQueryKey(params: FontStoreParams): readonly unknown[] {
+        const filtered: Record<string, any> = {};
+
+        for (const [key, value] of Object.entries(params)) {
+            // Ensure we DO NOT 'continue' or skip the limit key here.
+            // The limit is a fundamental part of the data identity.
+            if (
+                value !== undefined
+                && value !== null
+                && value !== ''
+                && !(Array.isArray(value) && value.length === 0)
+            ) {
+                filtered[key] = value;
+            }
+        }
+
+        return ['fonts', filtered];
+    }
+
+    private buildOptions(params = this.#params) {
+        const activeParams = { ...params };
+        const hasFilters = !!(
+            activeParams.q
+            || (Array.isArray(activeParams.providers) && activeParams.providers.length > 0)
+            || (Array.isArray(activeParams.categories) && activeParams.categories.length > 0)
+            || (Array.isArray(activeParams.subsets) && activeParams.subsets.length > 0)
+        );
+        return {
+            queryKey: this.buildQueryKey(activeParams),
+            queryFn: ({ pageParam }: QueryFunctionContext<readonly unknown[], PageParam>) =>
+                this.fetchPage({ ...activeParams, ...pageParam }),
+            initialPageParam: { offset: 0 } as PageParam,
+            getNextPageParam: (lastPage: ProxyFontsResponse): PageParam | undefined => {
+                const next = lastPage.offset + lastPage.limit;
+                return next < lastPage.total ? { offset: next } : undefined;
+            },
+            enabled: this.#enabled,
+            staleTime: hasFilters ? 0 : DEFAULT_QUERY_STALE_TIME_MS,
+            gcTime: DEFAULT_QUERY_GC_TIME_MS,
+        };
+    }
+
+    private async fetchPage(params: ProxyFontsParams): Promise<ProxyFontsResponse> {
+        let response: ProxyFontsResponse;
+        try {
+            response = await fetchProxyFonts(params);
+        } catch (cause) {
+            // Preserve non-retryable validation errors so the query client doesn't
+            // burn the retry budget on a deterministic schema mismatch.
+            if (cause instanceof FontResponseError) {
+                throw cause;
+            }
+            throw new FontNetworkError(cause);
+        }
+
+        if (!response) {
+            throw new FontResponseError('response', response);
+        }
+        if (!response.fonts) {
+            throw new FontResponseError('response.fonts', response.fonts);
+        }
+        if (!Array.isArray(response.fonts)) {
+            throw new FontResponseError('response.fonts', response.fonts);
+        }
+
+        return {
+            fonts: response.fonts,
+            total: response.total ?? 0,
+            limit: response.limit ?? params.limit ?? 50,
+            offset: response.offset ?? params.offset ?? 0,
+        };
+    }
+}
+
+const catalog = createSingleton(
+    () => new FontCatalogStore({ limit: 50 }),
+    instance => instance.destroy(),
+);
+
+export const getFontCatalog = catalog.get;
+
+// test-only reset, so specs don't share a live observer
+export const __resetFontCatalog = catalog.reset;

src/entities/Font/model/store/fontLifecycleManager/errors.ts

@@ -0,0 +1,35 @@
+/**
+ * Thrown by {@link FontBufferCache} when a font file cannot be retrieved from the network or cache.
+ *
+ * @property url - The URL that was requested.
+ * @property cause - The underlying error, if any.
+ * @property status - HTTP status code. Present on HTTP errors, absent on network failures.
+ */
+export class FontFetchError extends Error {
+    readonly name = 'FontFetchError';
+
+    constructor(
+        public readonly url: string,
+        public readonly cause?: unknown,
+        public readonly status?: number,
+    ) {
+        super(status ? `HTTP ${status} fetching font: ${url}` : `Network error fetching font: ${url}`);
+    }
+}
+
+/**
+ * Thrown by {@link loadFont} when a font buffer cannot be parsed into a {@link FontFace}.
+ *
+ * @property fontName - The display name of the font that failed to parse.
+ * @property cause - The underlying error from the FontFace API.
+ */
+export class FontParseError extends Error {
+    readonly name = 'FontParseError';
+
+    constructor(
+        public readonly fontName: string,
+        public readonly cause?: unknown,
+    ) {
+        super(`Failed to parse font: ${fontName}`);
+    }
+}

src/entities/Font/model/store/fontLifecycleManager/fontLifecycleManager.svelte.ts

@@ -0,0 +1,435 @@
+import { createSingleton } from '$shared/lib/helpers/createSingleton/createSingleton';
+import { SvelteMap } from 'svelte/reactivity';
+import {
+    type FontLoadRequestConfig,
+    type FontLoadStatus,
+} from '../../types';
+import {
+    FontFetchError,
+    FontParseError,
+} from './errors';
+import {
+    generateFontKey,
+    getEffectiveConcurrency,
+    loadFont,
+    yieldToMainThread,
+} from './utils';
+import { FontBufferCache } from './utils/fontBufferCache/FontBufferCache';
+import { FontEvictionPolicy } from './utils/fontEvictionPolicy/FontEvictionPolicy';
+import { FontLoadQueue } from './utils/fontLoadQueue/FontLoadQueue';
+
+/**
+ * How often the periodic eviction sweep runs.
+ */
+const PURGE_INTERVAL_MS = 60000;
+
+/**
+ * Timeout for `requestIdleCallback`. After this elapses, the callback is
+ * forced to run regardless of whether the browser is idle.
+ */
+const IDLE_CALLBACK_TIMEOUT_MS = 150;
+
+/**
+ * setTimeout fallback delay when `requestIdleCallback` is unavailable.
+ * ~16ms ≈ one frame at 60fps.
+ */
+const SCHEDULE_FALLBACK_MS = 16;
+
+/**
+ * How often the parse loop yields back to the main thread when the browser
+ * does not provide `isInputPending` (non-Chromium fallback).
+ */
+const YIELD_INTERVAL_MS = 8;
+
+/**
+ * Font weights treated as "critical" in data-saver mode. Other weights are
+ * skipped to reduce network usage; variable fonts bypass this filter.
+ */
+const CRITICAL_FONT_WEIGHTS = [400, 700];
+
+interface FontLifecycleManagerDeps {
+    cache?: FontBufferCache;
+    eviction?: FontEvictionPolicy;
+    queue?: FontLoadQueue;
+}
+
+/**
+ * Manages web font loading with caching, adaptive concurrency, and automatic cleanup.
+ *
+ * **Two-Phase Loading Strategy:**
+ * 1. *Concurrent Fetching*: Font files fetched in parallel (network I/O is non-blocking)
+ * 2. *Sequential Parsing*: Buffers parsed into FontFace objects one at a time with periodic yields
+ *
+ * **Yielding Strategy:**
+ * - Chromium: Yields only when user input is pending (via `scheduler.yield()` + `isInputPending()`)
+ * - Others: Time-based fallback, yields every 8ms
+ *
+ * **Network Adaptation:**
+ * - 2G: 1 concurrent request, 3G: 2, 4G+: 4 (via Network Information API)
+ * - Respects `saveData` mode to defer non-critical weights
+ *
+ * **Cache Integration:** Cache API with best-effort fallback (handles private browsing, quota limits)
+ *
+ * **Cleanup:** LRU-style eviction after 5 minutes of inactivity; cleanup runs every 60 seconds
+ *
+ * **Font Identity:** Variable fonts use `{id}@vf`, static fonts use `{id}@{weight}`
+ *
+ * **Browser APIs Used:** `scheduler.yield()`, `isInputPending()`, `requestIdleCallback`, Cache API, Network Information API
+ */
+export class FontLifecycleManager {
+    // Injected collaborators - each handles one concern for better testability
+    readonly #cache: FontBufferCache;
+    readonly #eviction: FontEvictionPolicy;
+    readonly #queue: FontLoadQueue;
+
+    // Loaded FontFace instances registered with document.fonts. Key: `{id}@{weight}` or `{id}@vf`
+    #loadedFonts = new Map<string, FontFace>();
+
+    // Maps font key → URL so #purgeUnused() can evict from cache
+    #urlByKey = new Map<string, string>();
+
+    // Handle for scheduled queue processing (requestIdleCallback or setTimeout)
+    #timeoutId: ReturnType<typeof setTimeout> | null = null;
+
+    // Interval handle for periodic cleanup (runs every PURGE_INTERVAL)
+    #intervalId: ReturnType<typeof setInterval> | null = null;
+
+    // AbortController for canceling in-flight fetches on destroy
+    #abortController = new AbortController();
+
+    // Tracks which callback type is pending ('idle' | 'timeout' | null) for proper cancellation
+    #pendingType: 'idle' | 'timeout' | null = null;
+
+    // Reactive status map for Svelte components to track font states
+    statuses = new SvelteMap<string, FontLoadStatus>();
+
+    // Starts periodic cleanup timer (browser-only).
+    constructor(
+        { cache = new FontBufferCache(), eviction = new FontEvictionPolicy(), queue = new FontLoadQueue() }:
+            FontLifecycleManagerDeps = {},
+    ) {
+        // Inject collaborators - defaults provided for production, fakes for testing
+        this.#cache = cache;
+        this.#eviction = eviction;
+        this.#queue = queue;
+        if (typeof window !== 'undefined') {
+            this.#intervalId = setInterval(() => this.#purgeUnused(), PURGE_INTERVAL_MS);
+        }
+    }
+
+    /**
+     * Requests fonts to be loaded. Updates usage tracking and queues new fonts.
+     *
+     * Retry behavior: 'loaded' and 'loading' fonts are skipped; 'error' fonts retry if count < MAX_RETRIES.
+     * Scheduling: Prefers requestIdleCallback (150ms timeout), falls back to setTimeout(16ms).
+     */
+    touch(configs: FontLoadRequestConfig[]) {
+        if (this.#abortController.signal.aborted) {
+            return;
+        }
+        try {
+            const now = Date.now();
+            let hasNewItems = false;
+
+            for (const config of configs) {
+                const key = generateFontKey(config);
+
+                // Update last-used timestamp for LRU eviction policy
+                this.#eviction.touch(key, now);
+
+                const status = this.statuses.get(key);
+
+                // Skip fonts that are already loaded or currently loading
+                if (status === 'loaded' || status === 'loading') {
+                    continue;
+                }
+
+                // Skip fonts already in the queue (avoid duplicates)
+                if (this.#queue.has(key)) {
+                    continue;
+                }
+
+                // Skip error fonts that have exceeded max retry count
+                if (status === 'error' && this.#queue.isMaxRetriesReached(key)) {
+                    continue;
+                }
+
+                // Queue this font for loading
+                this.#queue.enqueue(key, config);
+                hasNewItems = true;
+            }
+
+            if (hasNewItems && !this.#timeoutId) {
+                this.#scheduleProcessing();
+            }
+        } catch (error) {
+            console.error(error);
+        }
+    }
+
+    /**
+     * Schedules `#processQueue()` via `requestIdleCallback` (150ms timeout) when available,
+     * falling back to `setTimeout(16ms)` for ~60fps timing.
+     */
+    #scheduleProcessing(): void {
+        if (typeof requestIdleCallback !== 'undefined') {
+            this.#timeoutId = requestIdleCallback(
+                () => this.#processQueue(),
+                { timeout: IDLE_CALLBACK_TIMEOUT_MS },
+            ) as unknown as ReturnType<typeof setTimeout>;
+            this.#pendingType = 'idle';
+        } else {
+            this.#timeoutId = setTimeout(() => this.#processQueue(), SCHEDULE_FALLBACK_MS);
+            this.#pendingType = 'timeout';
+        }
+    }
+
+    /**
+     * Returns true if data-saver mode is enabled (defers non-critical weights).
+     */
+    #shouldDeferNonCritical(): boolean {
+        return (navigator as any).connection?.saveData === true;
+    }
+
+    /**
+     * Processes queued fonts in two phases:
+     * 1. Concurrent fetching (network I/O, non-blocking)
+     * 2. Sequential parsing with periodic yields (CPU-intensive, can block UI)
+     *
+     * Yielding: Chromium uses `isInputPending()` for optimal responsiveness; others yield every 8ms.
+     */
+    async #processQueue() {
+        // Clear timer flags since we're now processing
+        this.#timeoutId = null;
+        this.#pendingType = null;
+
+        // Get all queued entries and clear the queue atomically
+        let entries = this.#queue.flush();
+        if (!entries.length) {
+            return;
+        }
+
+        // In data-saver mode, only load variable fonts and common weights (400, 700)
+        if (this.#shouldDeferNonCritical()) {
+            entries = entries.filter(([, c]) => c.isVariable || CRITICAL_FONT_WEIGHTS.includes(c.weight));
+        }
+
+        // Determine optimal concurrent fetches based on network speed (1-4)
+        const concurrency = getEffectiveConcurrency();
+        const buffers = new Map<string, ArrayBuffer>();
+
+        // Fetch multiple font files in parallel since network I/O is non-blocking
+        for (let i = 0; i < entries.length; i += concurrency) {
+            await this.#fetchChunk(entries.slice(i, i + concurrency), buffers);
+        }
+
+        // Parse buffers one at a time with periodic yields to avoid blocking UI
+        const hasInputPending = !!(navigator as any).scheduling?.isInputPending;
+        let lastYield = performance.now();
+
+        for (const [key, config] of entries) {
+            const buffer = buffers.get(key);
+            // Skip fonts that failed to fetch in phase 1
+            if (!buffer) {
+                continue;
+            }
+
+            await this.#processFont(key, config, buffer);
+
+            // Yield to main thread if needed (prevents UI blocking)
+            // Chromium: use isInputPending() for optimal responsiveness
+            // Others: yield every 8ms as fallback
+            const shouldYield = hasInputPending
+                ? (navigator as any).scheduling.isInputPending({ includeContinuous: true })
+                : performance.now() - lastYield > YIELD_INTERVAL_MS;
+
+            if (shouldYield) {
+                await yieldToMainThread();
+                lastYield = performance.now();
+            }
+        }
+    }
+
+    /**
+     * Fetches a chunk of fonts concurrently and populates `buffers` with successful results.
+     * Each promise carries its own key and config so results need no index correlation.
+     * Aborted fetches are silently skipped; other errors set status to `'error'` and increment retry.
+     */
+    async #fetchChunk(
+        chunk: Array<[string, FontLoadRequestConfig]>,
+        buffers: Map<string, ArrayBuffer>,
+    ): Promise<void> {
+        const results = await Promise.all(
+            chunk.map(async ([key, config]) => {
+                this.statuses.set(key, 'loading');
+                try {
+                    const buffer = await this.#cache.get(config.url, this.#abortController.signal);
+                    buffers.set(key, buffer);
+                    return { ok: true as const, key };
+                } catch (reason) {
+                    return { ok: false as const, key, config, reason };
+                }
+            }),
+        );
+
+        for (const result of results) {
+            if (result.ok) {
+                continue;
+            }
+            const { key, config, reason } = result;
+            const isAbort = reason instanceof FontFetchError
+                && reason.cause instanceof Error
+                && reason.cause.name === 'AbortError';
+            if (isAbort) {
+                continue;
+            }
+            if (reason instanceof FontFetchError) {
+                console.error(`Font fetch failed: ${config.name}`, reason);
+            }
+            this.statuses.set(key, 'error');
+            this.#queue.incrementRetry(key);
+        }
+    }
+
+    /**
+     * Parses a fetched buffer into a {@link FontFace}, registers it with `document.fonts`,
+     * and updates reactive status. On failure, sets status to `'error'` and increments the retry count.
+     */
+    async #processFont(key: string, config: FontLoadRequestConfig, buffer: ArrayBuffer): Promise<void> {
+        try {
+            const font = await loadFont(config, buffer);
+            this.#loadedFonts.set(key, font);
+            this.#urlByKey.set(key, config.url);
+            this.statuses.set(key, 'loaded');
+        } catch (e) {
+            if (e instanceof FontParseError) {
+                console.error(`Font parse failed: ${config.name}`, e);
+                this.statuses.set(key, 'error');
+                this.#queue.incrementRetry(key);
+            }
+        }
+    }
+
+    /**
+     * Removes fonts unused within TTL (LRU-style cleanup). Runs every PURGE_INTERVAL. Pinned fonts are never evicted.
+     */
+    #purgeUnused() {
+        const now = Date.now();
+        // Iterate through all tracked font keys
+        for (const key of this.#eviction.keys()) {
+            // Skip fonts that are still within TTL or are pinned
+            if (!this.#eviction.shouldEvict(key, now)) {
+                continue;
+            }
+
+            // Remove FontFace from document to free memory
+            const font = this.#loadedFonts.get(key);
+            if (font) {
+                document.fonts.delete(font);
+            }
+
+            // Evict from cache and cleanup URL mapping
+            const url = this.#urlByKey.get(key);
+            if (url) {
+                this.#cache.evict(url);
+                this.#urlByKey.delete(key);
+            }
+
+            // Clean up remaining state
+            this.#loadedFonts.delete(key);
+            this.statuses.delete(key);
+            this.#eviction.remove(key);
+        }
+    }
+
+    /**
+     * Returns current loading status for a font, or undefined if never requested.
+     */
+    getFontStatus(id: string, weight: number, isVariable = false) {
+        try {
+            return this.statuses.get(generateFontKey({ id, weight, isVariable }));
+        } catch (error) {
+            console.error(error);
+        }
+    }
+
+    /**
+     * Pins a font so it is never evicted by #purgeUnused(), regardless of TTL.
+     */
+    pin(id: string, weight: number, isVariable = false): void {
+        this.#eviction.pin(generateFontKey({ id, weight, isVariable }));
+    }
+
+    /**
+     * Unpins a font, allowing it to be evicted by #purgeUnused() once its TTL expires.
+     */
+    unpin(id: string, weight: number, isVariable = false): void {
+        this.#eviction.unpin(generateFontKey({ id, weight, isVariable }));
+    }
+
+    /**
+     * Waits for all fonts to finish loading using document.fonts.ready.
+     */
+    async ready(): Promise<void> {
+        if (typeof document === 'undefined') {
+            return;
+        }
+        try {
+            await document.fonts.ready;
+        } catch { /* document unloaded */ }
+    }
+
+    /**
+     * Aborts all operations, removes fonts from document, and clears state. Manager cannot be reused after.
+     */
+    destroy() {
+        // Abort all in-flight network requests
+        this.#abortController.abort();
+
+        // Cancel pending queue processing (idle callback or timeout)
+        if (this.#timeoutId !== null) {
+            if (this.#pendingType === 'idle' && typeof cancelIdleCallback !== 'undefined') {
+                cancelIdleCallback(this.#timeoutId as unknown as number);
+            } else {
+                clearTimeout(this.#timeoutId);
+            }
+            this.#timeoutId = null;
+            this.#pendingType = null;
+        }
+
+        // Stop periodic cleanup timer
+        if (this.#intervalId) {
+            clearInterval(this.#intervalId);
+            this.#intervalId = null;
+        }
+
+        // Remove all loaded fonts from document
+        if (typeof document !== 'undefined') {
+            for (const font of this.#loadedFonts.values()) {
+                document.fonts.delete(font);
+            }
+        }
+
+        // Clear all state and collaborators
+        this.#loadedFonts.clear();
+        this.#urlByKey.clear();
+        this.#cache.clear();
+        this.#eviction.clear();
+        this.#queue.clear();
+        this.statuses.clear();
+    }
+}
+
+/**
+ * App-wide font lifecycle manager, created on first access. Lazy so its
+ * AbortController / FontFace bookkeeping isn't set up at module load.
+ */
+const fontLifecycleManager = createSingleton(
+    () => new FontLifecycleManager(),
+    instance => instance.destroy(),
+);
+
+export const getFontLifecycleManager = fontLifecycleManager.get;
+
+// test-only reset, so specs don't share loaded-font/eviction state
+export const __resetFontLifecycleManager = fontLifecycleManager.reset;

src/entities/Font/model/store/fontLifecycleManager/fontLifecycleManager.test.ts

@@ -0,0 +1,318 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { FontFetchError } from './errors';
+import { FontLifecycleManager } from './fontLifecycleManager.svelte';
+import { FontEvictionPolicy } from './utils/fontEvictionPolicy/FontEvictionPolicy';
+
+class FakeBufferCache {
+    async get(_url: string): Promise<ArrayBuffer> {
+        return new ArrayBuffer(8);
+    }
+    evict(_url: string): void {}
+    clear(): void {}
+}
+
+/**
+ * Throws {@link FontFetchError} on every `get()` — simulates network/HTTP failure.
+ */
+class FailingBufferCache {
+    async get(url: string): Promise<never> {
+        throw new FontFetchError(url, new Error('network error'), 500);
+    }
+    evict(_url: string): void {}
+    clear(): void {}
+}
+
+const makeConfig = (id: string, overrides: Partial<{ weight: number; isVariable: boolean }> = {}) => ({
+    id,
+    name: id,
+    url: `https://example.com/${id}.woff2`,
+    weight: 400,
+    ...overrides,
+});
+
+describe('FontLifecycleManager', () => {
+    let manager: FontLifecycleManager;
+    let eviction: FontEvictionPolicy;
+    let mockFontFaceSet: { add: ReturnType<typeof vi.fn>; delete: ReturnType<typeof vi.fn> };
+
+    beforeEach(() => {
+        vi.useFakeTimers();
+        eviction = new FontEvictionPolicy({ ttl: 60000 });
+        mockFontFaceSet = { add: vi.fn(), delete: vi.fn() };
+
+        Object.defineProperty(document, 'fonts', {
+            value: mockFontFaceSet,
+            configurable: true,
+            writable: true,
+        });
+
+        const MockFontFace = vi.fn(function(this: any, name: string, buffer: BufferSource) {
+            this.name = name;
+            this.buffer = buffer;
+            this.load = vi.fn().mockResolvedValue(this);
+        });
+        vi.stubGlobal('FontFace', MockFontFace);
+
+        manager = new FontLifecycleManager({ cache: new FakeBufferCache() as any, eviction });
+    });
+
+    afterEach(() => {
+        vi.clearAllTimers();
+        vi.useRealTimers();
+        vi.unstubAllGlobals();
+    });
+
+    describe('touch()', () => {
+        it('queues and loads a new font', async () => {
+            manager.touch([makeConfig('roboto')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(manager.getFontStatus('roboto', 400)).toBe('loaded');
+        });
+
+        it('batches multiple fonts into a single queue flush', async () => {
+            manager.touch([makeConfig('lato'), makeConfig('inter')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(mockFontFaceSet.add).toHaveBeenCalledTimes(2);
+        });
+
+        it('skips fonts that are already loaded', async () => {
+            manager.touch([makeConfig('lato')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            manager.touch([makeConfig('lato')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(mockFontFaceSet.add).toHaveBeenCalledTimes(1);
+        });
+
+        it('skips fonts that are currently loading', async () => {
+            manager.touch([makeConfig('lato')]);
+            // simulate loading state before queue drains
+            manager.statuses.set('lato@400', 'loading');
+            manager.touch([makeConfig('lato')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(mockFontFaceSet.add).toHaveBeenCalledTimes(1);
+        });
+
+        it('skips fonts that have exhausted retries', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const failManager = new FontLifecycleManager({ cache: new FailingBufferCache() as any, eviction });
+
+            // exhaust all 3 retries
+            for (let i = 0; i < 3; i++) {
+                failManager.statuses.delete('broken@400');
+                failManager.touch([makeConfig('broken')]);
+                await vi.advanceTimersByTimeAsync(50);
+            }
+
+            failManager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(failManager.getFontStatus('broken', 400)).toBe('error');
+            expect(mockFontFaceSet.add).not.toHaveBeenCalled();
+            consoleSpy.mockRestore();
+        });
+
+        it('does nothing after manager is destroyed', async () => {
+            manager.destroy();
+            manager.touch([makeConfig('roboto')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(manager.statuses.size).toBe(0);
+        });
+    });
+
+    describe('queue processing', () => {
+        it('filters non-critical weights in data-saver mode', async () => {
+            (navigator as any).connection = { saveData: true };
+
+            manager.touch([
+                makeConfig('light', { weight: 300 }),
+                makeConfig('regular', { weight: 400 }),
+                makeConfig('bold', { weight: 700 }),
+            ]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(manager.getFontStatus('light', 300)).toBeUndefined();
+            expect(manager.getFontStatus('regular', 400)).toBe('loaded');
+            expect(manager.getFontStatus('bold', 700)).toBe('loaded');
+
+            delete (navigator as any).connection;
+        });
+
+        it('loads variable fonts in data-saver mode regardless of weight', async () => {
+            (navigator as any).connection = { saveData: true };
+
+            manager.touch([makeConfig('vf', { weight: 300, isVariable: true })]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(manager.getFontStatus('vf', 300, true)).toBe('loaded');
+
+            delete (navigator as any).connection;
+        });
+    });
+
+    describe('Phase 1 — fetch', () => {
+        it('sets status to error on fetch failure', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const failManager = new FontLifecycleManager({ cache: new FailingBufferCache() as any, eviction });
+
+            failManager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(failManager.getFontStatus('broken', 400)).toBe('error');
+            consoleSpy.mockRestore();
+        });
+
+        it('logs a console error on fetch failure', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const failManager = new FontLifecycleManager({ cache: new FailingBufferCache() as any, eviction });
+
+            failManager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(consoleSpy).toHaveBeenCalled();
+            consoleSpy.mockRestore();
+        });
+
+        it('does not set error status or log for aborted fetches', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const abortingCache = {
+                async get(url: string): Promise<never> {
+                    throw new FontFetchError(url, Object.assign(new Error('Aborted'), { name: 'AbortError' }));
+                },
+                evict() {},
+                clear() {},
+            };
+            const abortManager = new FontLifecycleManager({ cache: abortingCache as any, eviction });
+
+            abortManager.touch([makeConfig('aborted')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            // status is left as 'loading' (not 'error') — abort is not a retriable failure
+            expect(abortManager.getFontStatus('aborted', 400)).not.toBe('error');
+            expect(consoleSpy).not.toHaveBeenCalled();
+            consoleSpy.mockRestore();
+        });
+    });
+
+    describe('Phase 2 — parse', () => {
+        it('sets status to error on parse failure', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const FailingFontFace = vi.fn(function(this: any) {
+                this.load = vi.fn().mockRejectedValue(new Error('parse failed'));
+            });
+            vi.stubGlobal('FontFace', FailingFontFace);
+
+            manager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(manager.getFontStatus('broken', 400)).toBe('error');
+            consoleSpy.mockRestore();
+        });
+
+        it('logs a console error on parse failure', async () => {
+            const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+            const FailingFontFace = vi.fn(function(this: any) {
+                this.load = vi.fn().mockRejectedValue(new Error('parse failed'));
+            });
+            vi.stubGlobal('FontFace', FailingFontFace);
+
+            manager.touch([makeConfig('broken')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(consoleSpy).toHaveBeenCalled();
+            consoleSpy.mockRestore();
+        });
+    });
+
+    describe('#purgeUnused', () => {
+        it('evicts fonts after TTL expires', async () => {
+            manager.touch([makeConfig('ephemeral')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            await vi.advanceTimersByTimeAsync(61000);
+
+            expect(manager.getFontStatus('ephemeral', 400)).toBeUndefined();
+            expect(mockFontFaceSet.delete).toHaveBeenCalled();
+        });
+
+        it('removes the evicted key from the eviction policy', async () => {
+            manager.touch([makeConfig('ephemeral')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            await vi.advanceTimersByTimeAsync(61000);
+
+            expect(Array.from(eviction.keys())).not.toContain('ephemeral@400');
+        });
+
+        it('refreshes TTL when font is re-touched before expiry', async () => {
+            const config = makeConfig('active');
+            manager.touch([config]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            await vi.advanceTimersByTimeAsync(40000);
+            manager.touch([config]); // refresh at t≈40s
+
+            await vi.advanceTimersByTimeAsync(25000); // purge at t≈60s sees only ~20s elapsed → not evicted
+
+            expect(manager.getFontStatus('active', 400)).toBe('loaded');
+        });
+
+        it('does not evict pinned fonts', async () => {
+            manager.touch([makeConfig('pinned')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            manager.pin('pinned', 400);
+            await vi.advanceTimersByTimeAsync(61000);
+
+            expect(manager.getFontStatus('pinned', 400)).toBe('loaded');
+            expect(mockFontFaceSet.delete).not.toHaveBeenCalled();
+        });
+
+        it('evicts font after it is unpinned and TTL expires', async () => {
+            manager.touch([makeConfig('toggled')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            manager.pin('toggled', 400);
+            manager.unpin('toggled', 400);
+            await vi.advanceTimersByTimeAsync(61000);
+
+            expect(manager.getFontStatus('toggled', 400)).toBeUndefined();
+            expect(mockFontFaceSet.delete).toHaveBeenCalled();
+        });
+    });
+
+    describe('destroy()', () => {
+        it('clears all statuses', async () => {
+            manager.touch([makeConfig('roboto')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            manager.destroy();
+
+            expect(manager.statuses.size).toBe(0);
+        });
+
+        it('removes all loaded fonts from document.fonts', async () => {
+            manager.touch([makeConfig('roboto'), makeConfig('inter')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            manager.destroy();
+
+            expect(mockFontFaceSet.delete).toHaveBeenCalledTimes(2);
+        });
+
+        it('prevents further loading after destroy', async () => {
+            manager.destroy();
+            manager.touch([makeConfig('roboto')]);
+            await vi.advanceTimersByTimeAsync(50);
+
+            expect(manager.statuses.size).toBe(0);
+        });
+    });
+});

src/entities/Font/model/store/fontLifecycleManager/utils/fontBufferCache/FontBufferCache.test.ts

@@ -0,0 +1,68 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { FontFetchError } from '../../errors';
+import { FontBufferCache } from './FontBufferCache';
+
+const makeBuffer = () => new ArrayBuffer(8);
+
+const makeFetcher = (overrides: Partial<Response> = {}) =>
+    vi.fn().mockResolvedValue({
+        ok: true,
+        status: 200,
+        arrayBuffer: () => Promise.resolve(makeBuffer()),
+        clone: () => ({ ok: true, status: 200, arrayBuffer: () => Promise.resolve(makeBuffer()) }),
+        ...overrides,
+    } as Response);
+
+describe('FontBufferCache', () => {
+    let cache: FontBufferCache;
+    let fetcher: ReturnType<typeof makeFetcher>;
+
+    beforeEach(() => {
+        fetcher = makeFetcher();
+        cache = new FontBufferCache({ fetcher });
+    });
+
+    it('returns buffer from memory on second call without fetching', async () => {
+        await cache.get('https://example.com/font.woff2');
+        await cache.get('https://example.com/font.woff2');
+
+        expect(fetcher).toHaveBeenCalledOnce();
+    });
+
+    it('throws FontFetchError on HTTP error with correct status', async () => {
+        const errorFetcher = makeFetcher({ ok: false, status: 404 });
+        const errorCache = new FontBufferCache({ fetcher: errorFetcher });
+
+        const err = await errorCache.get('https://example.com/font.woff2').catch(e => e);
+        expect(err).toBeInstanceOf(FontFetchError);
+        expect(err.status).toBe(404);
+    });
+
+    it('throws FontFetchError on network failure without status', async () => {
+        const networkFetcher = vi.fn().mockRejectedValue(new Error('network down'));
+        const networkCache = new FontBufferCache({ fetcher: networkFetcher });
+
+        const err = await networkCache.get('https://example.com/font.woff2').catch(e => e);
+        expect(err).toBeInstanceOf(FontFetchError);
+        expect(err.status).toBeUndefined();
+    });
+
+    it('evict removes url from memory so next call fetches again', async () => {
+        await cache.get('https://example.com/font.woff2');
+        cache.evict('https://example.com/font.woff2');
+        await cache.get('https://example.com/font.woff2');
+
+        expect(fetcher).toHaveBeenCalledTimes(2);
+    });
+
+    it('clear wipes all memory cache entries', async () => {
+        await cache.get('https://example.com/a.woff2');
+        await cache.get('https://example.com/b.woff2');
+        cache.clear();
+        await cache.get('https://example.com/a.woff2');
+
+        expect(fetcher).toHaveBeenCalledTimes(3);
+    });
+});

src/entities/Font/model/store/fontLifecycleManager/utils/fontBufferCache/FontBufferCache.ts

@@ -0,0 +1,105 @@
+import { FontFetchError } from '../../errors';
+
+type Fetcher = (url: string, init?: RequestInit) => Promise<Response>;
+
+interface FontBufferCacheOptions {
+    /**
+     * Custom fetch implementation. Defaults to `globalThis.fetch`. Inject in tests for isolation.
+     */
+    fetcher?: Fetcher;
+    /**
+     * Cache API cache name. Defaults to `'font-cache-v1'`.
+     */
+    cacheName?: string;
+}
+
+/**
+ * Three-tier font buffer cache: in-memory → Cache API → network.
+ *
+ * - **Tier 1 (memory):** Fastest — no I/O. Populated after first successful fetch.
+ * - **Tier 2 (Cache API):** Persists across page loads. Silently skipped in private browsing.
+ * - **Tier 3 (network):** Raw fetch. Throws {@link FontFetchError} on failure.
+ *
+ * The `fetcher` option is injectable for testing — pass a `vi.fn()` to avoid real network calls.
+ */
+export class FontBufferCache {
+    #buffersByUrl = new Map<string, ArrayBuffer>();
+
+    readonly #fetcher: Fetcher;
+    readonly #cacheName: string;
+
+    constructor(
+        { fetcher = globalThis.fetch.bind(globalThis), cacheName = 'font-cache-v1' }: FontBufferCacheOptions = {},
+    ) {
+        this.#fetcher = fetcher;
+        this.#cacheName = cacheName;
+    }
+
+    /**
+     * Retrieves the font buffer for the given URL using the three-tier strategy.
+     * Stores the result in memory on success.
+     *
+     * @throws {@link FontFetchError} if the network request fails or returns a non-OK response.
+     */
+    async get(url: string, signal?: AbortSignal): Promise<ArrayBuffer> {
+        // Tier 1: in-memory (fastest, no I/O)
+        const inMemory = this.#buffersByUrl.get(url);
+        if (inMemory) {
+            return inMemory;
+        }
+
+        // Tier 2: Cache API
+        try {
+            if (typeof caches !== 'undefined') {
+                const cache = await caches.open(this.#cacheName);
+                const cached = await cache.match(url);
+                if (cached) {
+                    const buffer = await cached.arrayBuffer();
+                    this.#buffersByUrl.set(url, buffer);
+                    return buffer;
+                }
+            }
+        } catch {
+            // Cache unavailable (private browsing, security restrictions) — fall through to network
+        }
+
+        // Tier 3: network
+        let response: Response;
+        try {
+            response = await this.#fetcher(url, { signal });
+        } catch (cause) {
+            throw new FontFetchError(url, cause);
+        }
+
+        if (!response.ok) {
+            throw new FontFetchError(url, undefined, response.status);
+        }
+
+        try {
+            if (typeof caches !== 'undefined') {
+                const cache = await caches.open(this.#cacheName);
+                await cache.put(url, response.clone());
+            }
+        } catch {
+            // Cache write failed (quota, storage pressure) — return font anyway
+        }
+
+        const buffer = await response.arrayBuffer();
+        this.#buffersByUrl.set(url, buffer);
+        return buffer;
+    }
+
+    /**
+     * Removes a URL from the in-memory cache. Next call to `get()` will re-fetch.
+     */
+    evict(url: string): void {
+        this.#buffersByUrl.delete(url);
+    }
+
+    /**
+     * Clears all in-memory cached buffers.
+     */
+    clear(): void {
+        this.#buffersByUrl.clear();
+    }
+}

src/entities/Font/model/store/fontLifecycleManager/utils/fontEvictionPolicy/FontEvictionPolicy.test.ts

@@ -0,0 +1,69 @@
+import { FontEvictionPolicy } from './FontEvictionPolicy';
+
+describe('FontEvictionPolicy', () => {
+    let policy: FontEvictionPolicy;
+    const TTL = 1000;
+    const t0 = 100000;
+
+    beforeEach(() => {
+        policy = new FontEvictionPolicy({ ttl: TTL });
+    });
+
+    it('shouldEvict returns false within TTL', () => {
+        policy.touch('a@400', t0);
+        expect(policy.shouldEvict('a@400', t0 + TTL - 1)).toBe(false);
+    });
+
+    it('shouldEvict returns true at TTL boundary', () => {
+        policy.touch('a@400', t0);
+        expect(policy.shouldEvict('a@400', t0 + TTL)).toBe(true);
+    });
+
+    it('shouldEvict returns false for pinned key regardless of TTL', () => {
+        policy.touch('a@400', t0);
+        policy.pin('a@400');
+        expect(policy.shouldEvict('a@400', t0 + TTL * 10)).toBe(false);
+    });
+
+    it('shouldEvict returns true again after unpin past TTL', () => {
+        policy.touch('a@400', t0);
+        policy.pin('a@400');
+        policy.unpin('a@400');
+        expect(policy.shouldEvict('a@400', t0 + TTL)).toBe(true);
+    });
+
+    it('shouldEvict returns false for untracked key', () => {
+        expect(policy.shouldEvict('never@touched', t0 + TTL * 100)).toBe(false);
+    });
+
+    it('keys returns all tracked keys', () => {
+        policy.touch('a@400', t0);
+        policy.touch('b@vf', t0);
+        expect(Array.from(policy.keys())).toEqual(expect.arrayContaining(['a@400', 'b@vf']));
+    });
+
+    it('remove deletes key from tracking so it no longer appears in keys()', () => {
+        policy.touch('a@400', t0);
+        policy.touch('b@vf', t0);
+        policy.remove('a@400');
+        expect(Array.from(policy.keys())).not.toContain('a@400');
+        expect(Array.from(policy.keys())).toContain('b@vf');
+    });
+
+    it('remove unpins the key so a subsequent touch + TTL would evict it', () => {
+        policy.touch('a@400', t0);
+        policy.pin('a@400');
+        policy.remove('a@400');
+        // re-touch and check it can be evicted again
+        policy.touch('a@400', t0);
+        expect(policy.shouldEvict('a@400', t0 + TTL)).toBe(true);
+    });
+
+    it('clear resets all state', () => {
+        policy.touch('a@400', t0);
+        policy.pin('a@400');
+        policy.clear();
+        expect(Array.from(policy.keys())).toHaveLength(0);
+        expect(policy.shouldEvict('a@400', t0 + TTL * 10)).toBe(false);
+    });
+});

src/entities/Font/model/store/fontLifecycleManager/utils/fontEvictionPolicy/FontEvictionPolicy.ts

@@ -0,0 +1,93 @@
+/**
+ * Default TTL after which an unpinned font is eligible for eviction.
+ */
+export const DEFAULT_FONT_TTL_MS = 5 * 60 * 1000;
+
+interface FontEvictionPolicyOptions {
+    /**
+     * TTL in milliseconds. Defaults to {@link DEFAULT_FONT_TTL_MS}.
+     */
+    ttl?: number;
+}
+
+/**
+ * Tracks font usage timestamps and pinned keys to determine when a font should be evicted.
+ *
+ * Pure data — no browser APIs. Accepts explicit `now` timestamps so tests
+ * never need fake timers.
+ */
+export class FontEvictionPolicy {
+    #usageTracker = new Map<string, number>();
+    #pinnedFonts = new Set<string>();
+
+    readonly #TTL: number;
+
+    constructor({ ttl = DEFAULT_FONT_TTL_MS }: FontEvictionPolicyOptions = {}) {
+        this.#TTL = ttl;
+    }
+
+    /**
+     * Records the last-used time for a font key.
+     * @param key - Font key in `{id}@{weight}` or `{id}@vf` format.
+     * @param now - Current timestamp in ms. Defaults to `Date.now()`.
+     */
+    touch(key: string, now: number = Date.now()): void {
+        this.#usageTracker.set(key, now);
+    }
+
+    /**
+     * Pins a font key so it is never evicted regardless of TTL.
+     */
+    pin(key: string): void {
+        this.#pinnedFonts.add(key);
+    }
+
+    /**
+     * Unpins a font key, allowing it to be evicted once its TTL expires.
+     */
+    unpin(key: string): void {
+        this.#pinnedFonts.delete(key);
+    }
+
+    /**
+     * Returns `true` if the font should be evicted.
+     * A font is evicted when its TTL has elapsed and it is not pinned.
+     * Returns `false` for untracked keys.
+     *
+     * @param key - Font key to check.
+     * @param now - Current timestamp in ms (pass explicitly for deterministic tests).
+     */
+    shouldEvict(key: string, now: number): boolean {
+        const lastUsed = this.#usageTracker.get(key);
+        if (lastUsed === undefined) {
+            return false;
+        }
+        if (this.#pinnedFonts.has(key)) {
+            return false;
+        }
+        return now - lastUsed >= this.#TTL;
+    }
+
+    /**
+     * Returns an iterator over all tracked font keys.
+     */
+    keys(): IterableIterator<string> {
+        return this.#usageTracker.keys();
+    }
+
+    /**
+     * Removes a font key from tracking. Called by the orchestrator after eviction.
+     */
+    remove(key: string): void {
+        this.#usageTracker.delete(key);
+        this.#pinnedFonts.delete(key);
+    }
+
+    /**
+     * Clears all usage timestamps and pinned keys.
+     */
+    clear(): void {
+        this.#usageTracker.clear();
+        this.#pinnedFonts.clear();
+    }
+}

src/entities/Font/model/store/fontLifecycleManager/utils/fontLoadQueue/FontLoadQueue.test.ts

@@ -0,0 +1,65 @@
+import type { FontLoadRequestConfig } from '../../../../types';
+import { FontLoadQueue } from './FontLoadQueue';
+
+const config = (id: string): FontLoadRequestConfig => ({
+    id,
+    name: id,
+    url: `https://example.com/${id}.woff2`,
+    weight: 400,
+});
+
+describe('FontLoadQueue', () => {
+    let queue: FontLoadQueue;
+
+    beforeEach(() => {
+        queue = new FontLoadQueue();
+    });
+
+    it('enqueue returns true for a new key', () => {
+        expect(queue.enqueue('a@400', config('a'))).toBe(true);
+    });
+
+    it('enqueue returns false for an already-queued key', () => {
+        queue.enqueue('a@400', config('a'));
+        expect(queue.enqueue('a@400', config('a'))).toBe(false);
+    });
+
+    it('has returns true after enqueue, false after flush', () => {
+        queue.enqueue('a@400', config('a'));
+        expect(queue.has('a@400')).toBe(true);
+        queue.flush();
+        expect(queue.has('a@400')).toBe(false);
+    });
+
+    it('flush returns all entries and atomically clears the queue', () => {
+        queue.enqueue('a@400', config('a'));
+        queue.enqueue('b@700', config('b'));
+        const entries = queue.flush();
+        expect(entries).toHaveLength(2);
+        expect(queue.has('a@400')).toBe(false);
+        expect(queue.has('b@700')).toBe(false);
+    });
+
+    it('isMaxRetriesReached returns false below MAX_RETRIES', () => {
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        expect(queue.isMaxRetriesReached('a@400')).toBe(false);
+    });
+
+    it('isMaxRetriesReached returns true at MAX_RETRIES (3)', () => {
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        expect(queue.isMaxRetriesReached('a@400')).toBe(true);
+    });
+
+    it('clear resets queue and retry counts', () => {
+        queue.enqueue('a@400', config('a'));
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        queue.incrementRetry('a@400');
+        queue.clear();
+        expect(queue.has('a@400')).toBe(false);
+        expect(queue.isMaxRetriesReached('a@400')).toBe(false);
+    });
+});

src/entities/Font/model/store/fontLifecycleManager/utils/fontLoadQueue/FontLoadQueue.ts

@@ -0,0 +1,69 @@
+import type { FontLoadRequestConfig } from '../../../../types';
+
+/**
+ * Maximum number of times a single font key will be retried before it is
+ * considered permanently failed.
+ */
+export const FONT_LOAD_MAX_RETRIES = 3;
+
+/**
+ * Manages the font load queue and per-font retry counts.
+ *
+ * Scheduling (when to drain the queue) is handled by the orchestrator —
+ * this class is purely concerned with what is queued and whether retries are exhausted.
+ */
+export class FontLoadQueue {
+    #queue = new Map<string, FontLoadRequestConfig>();
+    #retryCounts = new Map<string, number>();
+
+    /**
+     * Adds a font to the queue.
+     * @returns `true` if the key was newly enqueued, `false` if it was already present.
+     */
+    enqueue(key: string, config: FontLoadRequestConfig): boolean {
+        if (this.#queue.has(key)) {
+            return false;
+        }
+        this.#queue.set(key, config);
+        return true;
+    }
+
+    /**
+     * Atomically snapshots and clears the queue.
+     * @returns All queued entries at the time of the call.
+     */
+    flush(): Array<[string, FontLoadRequestConfig]> {
+        const entries = Array.from(this.#queue.entries());
+        this.#queue.clear();
+        return entries;
+    }
+
+    /**
+     * Returns `true` if the key is currently in the queue.
+     */
+    has(key: string): boolean {
+        return this.#queue.has(key);
+    }
+
+    /**
+     * Increments the retry count for a font key.
+     */
+    incrementRetry(key: string): void {
+        this.#retryCounts.set(key, (this.#retryCounts.get(key) ?? 0) + 1);
+    }
+
+    /**
+     * Returns `true` if the font has reached or exceeded the maximum retry limit.
+     */
+    isMaxRetriesReached(key: string): boolean {
+        return (this.#retryCounts.get(key) ?? 0) >= FONT_LOAD_MAX_RETRIES;
+    }
+
+    /**
+     * Clears all queued fonts and resets all retry counts.
+     */
+    clear(): void {
+        this.#queue.clear();
+        this.#retryCounts.clear();
+    }
+}

src/entities/Font/model/store/fontLifecycleManager/utils/generateFontKey/generateFontKey.test.ts

@@ -0,0 +1,25 @@
+import { generateFontKey } from './generateFontKey';
+
+describe('generateFontKey', () => {
+    it('should throw an error if font id is not provided', () => {
+        const config = { weight: 400, isVariable: false };
+        // @ts-expect-error
+        expect(() => generateFontKey(config)).toThrow('Font id is required');
+    });
+
+    it('should generate a font key for a variable font', () => {
+        const config = { id: 'Roboto', weight: 400, isVariable: true };
+        expect(generateFontKey(config)).toBe('roboto@vf');
+    });
+
+    it('should throw an error if font weight is not provided and is not a variable font', () => {
+        const config = { id: 'Roboto', isVariable: false };
+        // @ts-expect-error
+        expect(() => generateFontKey(config)).toThrow('Font weight is required');
+    });
+
+    it('should generate a font key for a non-variable font', () => {
+        const config = { id: 'Roboto', weight: 400, isVariable: false };
+        expect(generateFontKey(config)).toBe('roboto@400');
+    });
+});

src/entities/Font/model/store/fontLifecycleManager/utils/generateFontKey/generateFontKey.ts

@@ -0,0 +1,22 @@
+import type { FontLoadRequestConfig } from '../../../../types';
+
+export type PartialConfig = Pick<FontLoadRequestConfig, 'id' | 'weight' | 'isVariable'>;
+
+/**
+ * Generates a font key for a given font load request configuration.
+ * @param config - The font load request configuration.
+ * @returns The generated font key.
+ */
+export function generateFontKey(config: PartialConfig): string {
+    if (!config.id) {
+        throw new Error('Font id is required');
+    }
+    if (config.isVariable) {
+        return `${config.id.toLowerCase()}@vf`;
+    }
+
+    if (!config.weight) {
+        throw new Error('Font weight is required');
+    }
+    return `${config.id.toLowerCase()}@${config.weight}`;
+}

src/entities/Font/model/store/fontLifecycleManager/utils/getEffectiveConcurrency/getEffectiveConcurrency.test.ts

@@ -0,0 +1,41 @@
+import {
+    beforeEach,
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import {
+    Concurrency,
+    getEffectiveConcurrency,
+} from './getEffectiveConcurrency';
+
+describe('getEffectiveConcurrency', () => {
+    beforeEach(() => {
+        const nav = navigator as any;
+        nav.connection = null;
+    });
+
+    it('should return MAX when connection is not available', () => {
+        const nav = navigator as any;
+        nav.connection = null;
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MAX);
+    });
+
+    it('should return MIN for slow-2g or 2g connection', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: 'slow-2g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MIN);
+    });
+
+    it('should return AVERAGE for 3g connection', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: '3g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.AVERAGE);
+    });
+
+    it('should return MAX for other connection types', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: '4g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MAX);
+    });
+});

src/entities/Font/model/store/fontLifecycleManager/utils/getEffectiveConcurrency/getEffectiveConcurrency.ts

@@ -0,0 +1,26 @@
+export enum Concurrency {
+    MIN = 1,
+    AVERAGE = 2,
+    MAX = 4,
+}
+
+/**
+ * Calculates the amount of fonts for concurrent download based on the user internet connection
+ */
+export function getEffectiveConcurrency(): number {
+    const nav = navigator as any;
+    const connection = nav.connection;
+    if (!connection) {
+        return Concurrency.MAX;
+    }
+
+    switch (connection.effectiveType) {
+        case 'slow-2g':
+        case '2g':
+            return Concurrency.MIN;
+        case '3g':
+            return Concurrency.AVERAGE;
+        default:
+            return Concurrency.MAX;
+    }
+}

src/entities/Font/model/store/fontLifecycleManager/utils/index.ts

@@ -0,0 +1,4 @@
+export { generateFontKey } from './generateFontKey/generateFontKey';
+export { getEffectiveConcurrency } from './getEffectiveConcurrency/getEffectiveConcurrency';
+export { loadFont } from './loadFont/loadFont';
+export { yieldToMainThread } from './yieldToMainThread/yieldToMainThread';

src/entities/Font/model/store/fontLifecycleManager/utils/loadFont/loadFont.test.ts

@@ -0,0 +1,95 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { FontParseError } from '../../errors';
+import { loadFont } from './loadFont';
+
+describe('loadFont', () => {
+    let mockFontInstance: any;
+    let mockFontFaceSet: { add: ReturnType<typeof vi.fn>; delete: ReturnType<typeof vi.fn> };
+
+    beforeEach(() => {
+        mockFontFaceSet = { add: vi.fn(), delete: vi.fn() };
+        Object.defineProperty(document, 'fonts', { value: mockFontFaceSet, configurable: true, writable: true });
+
+        const MockFontFace = vi.fn(
+            function(this: any, name: string, buffer: BufferSource, options: FontFaceDescriptors) {
+                this.name = name;
+                this.buffer = buffer;
+                this.options = options;
+                this.load = vi.fn().mockResolvedValue(this);
+                mockFontInstance = this;
+            },
+        );
+        vi.stubGlobal('FontFace', MockFontFace);
+    });
+
+    afterEach(() => {
+        vi.unstubAllGlobals();
+    });
+
+    it('constructs FontFace with exact weight for static fonts', async () => {
+        const buffer = new ArrayBuffer(8);
+        await loadFont({ name: 'Roboto', weight: 400 }, buffer);
+
+        expect(FontFace).toHaveBeenCalledWith('Roboto', buffer, expect.objectContaining({ weight: '400' }));
+    });
+
+    it('constructs FontFace with weight range for variable fonts', async () => {
+        const buffer = new ArrayBuffer(8);
+        await loadFont({ name: 'Roboto', weight: 400, isVariable: true }, buffer);
+
+        expect(FontFace).toHaveBeenCalledWith('Roboto', buffer, expect.objectContaining({ weight: '100 900' }));
+    });
+
+    it('sets style: normal and display: swap on FontFace options', async () => {
+        await loadFont({ name: 'Lato', weight: 700 }, new ArrayBuffer(8));
+
+        expect(FontFace).toHaveBeenCalledWith(
+            'Lato',
+            expect.anything(),
+            expect.objectContaining({ style: 'normal', display: 'swap' }),
+        );
+    });
+
+    it('passes the buffer as the second argument to FontFace', async () => {
+        const buffer = new ArrayBuffer(16);
+        await loadFont({ name: 'Inter', weight: 400 }, buffer);
+
+        expect(FontFace).toHaveBeenCalledWith('Inter', buffer, expect.anything());
+    });
+
+    it('calls font.load() and adds the font to document.fonts', async () => {
+        const buffer = new ArrayBuffer(8);
+        const result = await loadFont({ name: 'Inter', weight: 400 }, buffer);
+
+        expect(mockFontInstance.load).toHaveBeenCalledOnce();
+        expect(mockFontFaceSet.add).toHaveBeenCalledWith(mockFontInstance);
+        expect(result).toBe(mockFontInstance);
+    });
+
+    it('throws FontParseError when font.load() rejects', async () => {
+        const loadError = new Error('parse failed');
+        const MockFontFace = vi.fn(
+            function(this: any, _name: string, _buffer: BufferSource, _options: FontFaceDescriptors) {
+                this.load = vi.fn().mockRejectedValue(loadError);
+            },
+        );
+        vi.stubGlobal('FontFace', MockFontFace);
+
+        await expect(loadFont({ name: 'Broken', weight: 400 }, new ArrayBuffer(8))).rejects.toBeInstanceOf(
+            FontParseError,
+        );
+    });
+
+    it('throws FontParseError when document.fonts.add throws', async () => {
+        const addError = new Error('add failed');
+        mockFontFaceSet.add.mockImplementation(() => {
+            throw addError;
+        });
+
+        await expect(loadFont({ name: 'Broken', weight: 400 }, new ArrayBuffer(8))).rejects.toBeInstanceOf(
+            FontParseError,
+        );
+    });
+});

src/entities/Font/model/store/fontLifecycleManager/utils/loadFont/loadFont.ts

@@ -0,0 +1,27 @@
+import type { FontLoadRequestConfig } from '../../../../types';
+import { FontParseError } from '../../errors';
+
+export type PartialConfig = Pick<FontLoadRequestConfig, 'weight' | 'name' | 'isVariable'>;
+/**
+ * Loads a font from a buffer and adds it to the document's font collection.
+ * @param config - The font load request configuration.
+ * @param buffer - The buffer containing the font data.
+ * @returns A promise that resolves to the loaded `FontFace`.
+ * @throws {@link FontParseError} When the font buffer cannot be parsed or added to the document font set.
+ */
+export async function loadFont(config: PartialConfig, buffer: BufferSource): Promise<FontFace> {
+    try {
+        const weightRange = config.isVariable ? '100 900' : `${config.weight}`;
+        const font = new FontFace(config.name, buffer, {
+            weight: weightRange,
+            style: 'normal',
+            display: 'swap',
+        });
+        await font.load();
+        document.fonts.add(font);
+
+        return font;
+    } catch (error) {
+        throw new FontParseError(config.name, error);
+    }
+}

src/entities/Font/model/store/fontLifecycleManager/utils/yieldToMainThread/yieldToMainThread.test.ts

@@ -0,0 +1,17 @@
+import { yieldToMainThread } from './yieldToMainThread';
+
+describe('yieldToMainThread', () => {
+    it('uses scheduler.yield when available', async () => {
+        const mockYield = vi.fn().mockResolvedValue(undefined);
+        vi.stubGlobal('scheduler', { yield: mockYield });
+
+        await yieldToMainThread();
+
+        expect(mockYield).toHaveBeenCalledOnce();
+        vi.unstubAllGlobals();
+    });
+    it('falls back to MessageChannel when scheduler is unavailable', async () => {
+        // scheduler is not defined in jsdom by default
+        await expect(yieldToMainThread()).resolves.toBeUndefined();
+    });
+});

src/entities/Font/model/store/fontLifecycleManager/utils/yieldToMainThread/yieldToMainThread.ts

@@ -0,0 +1,14 @@
+/**
+ * Yields to main thread during CPU-intensive parsing. Uses scheduler.yield() where available or MessageChannel fallback.
+ */
+export async function yieldToMainThread(): Promise<void> {
+    if (typeof scheduler !== 'undefined' && 'yield' in scheduler) {
+        await scheduler.yield();
+    } else {
+        await new Promise<void>(resolve => {
+            const ch = new MessageChannel();
+            ch.port1.onmessage = () => resolve();
+            ch.port2.postMessage(null);
+        });
+    }
+}