refactor: extract magic constants — wave 1 (UX, API, storage)

- Use existing MULTIPLIER_S/M/L from \$entities/Font in SliderArea instead
  of inlining the 0.5/0.75/1 literals (constants already existed but were
  duplicated at the call site).
- Centralize API base URL in \$shared/api/endpoints.ts (was duplicated
  between proxyFonts and FilterAndSortFonts filters api).
- Promote every 'glyphdiff:...' localStorage key to a named module-level
  STORAGE_KEY constant. Test files now import the source constant rather
  than redeclaring it (eliminates silent-typo divergence risk).

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

@@ -29,10 +29,12 @@ export function seedFontCache(fonts: UnifiedFont[]): void {
     });
 }
 
+import { API_ENDPOINTS } from '$shared/api/endpoints';
+
 /**
- * Proxy API base URL
+ * Proxy API endpoint for font resources.
  */
-const PROXY_API_URL = 'https://api.glyphdiff.com/api/v1/fonts' as const;
+const PROXY_API_URL = API_ENDPOINTS.fonts;
 
 /**
  * Proxy API parameters

src/features/AdjustTypography/model/store/typographySettingsStore/typographySettingsStore.svelte.ts

@@ -297,6 +297,16 @@ export class TypographySettingsStore {
     }
 }
 
+/**
+ * Default factory storage key — used when a caller doesn't pass one.
+ */
+const DEFAULT_STORAGE_KEY = 'glyphdiff:typography';
+
+/**
+ * Storage key used by the app-wide singleton (scoped to comparison view).
+ */
+const COMPARISON_STORAGE_KEY = 'glyphdiff:comparison:typography';
+
 /**
  * Creates a typography control manager
  *
@@ -306,7 +316,7 @@ export class TypographySettingsStore {
  */
 export function createTypographySettingsStore(
     configs: ControlModel<ControlId>[],
-    storageId: string = 'glyphdiff:typography',
+    storageId: string = DEFAULT_STORAGE_KEY,
 ) {
     const storage = createPersistentStore<TypographySettings>(storageId, {
         fontSize: DEFAULT_FONT_SIZE,
@@ -322,5 +332,5 @@ export function createTypographySettingsStore(
  */
 export const typographySettingsStore = createTypographySettingsStore(
     DEFAULT_TYPOGRAPHY_CONTROLS_DATA,
-    'glyphdiff:comparison:typography',
+    COMPARISON_STORAGE_KEY,
 );

src/features/ChangeAppTheme/model/store/ThemeManager/ThemeManager.svelte.ts

@@ -30,6 +30,8 @@
 
 import { createPersistentStore } from '$shared/lib';
 
+export const STORAGE_KEY = 'glyphdiff:theme';
+
 type Theme = 'light' | 'dark';
 type ThemeSource = 'system' | 'user';
 
@@ -56,7 +58,7 @@ class ThemeManager {
     /**
      * Persistent storage for user's theme preference
      */
-    #store = createPersistentStore<Theme | null>('glyphdiff:theme', null);
+    #store = createPersistentStore<Theme | null>(STORAGE_KEY, null);
     /**
      * Bound handler for system theme change events
      */

src/features/ChangeAppTheme/model/store/ThemeManager/ThemeManager.test.ts

@@ -40,8 +40,7 @@ import { ThemeManager } from './ThemeManager.svelte';
  * - MediaQueryList listener management
  */
 
-// Storage key used by ThemeManager
+import { STORAGE_KEY } from './ThemeManager.svelte';
-const STORAGE_KEY = 'glyphdiff:theme';
 
 // Helper type for MediaQueryList event handler
 type MediaQueryListCallback = (this: MediaQueryList, ev: MediaQueryListEvent) => void;

src/features/FilterAndSortFonts/api/filters/filters.ts

@@ -8,8 +8,9 @@
  */
 
 import { api } from '$shared/api/api';
+import { API_ENDPOINTS } from '$shared/api/endpoints';
 
-const PROXY_API_URL = 'https://api.glyphdiff.com/api/v1/filters' as const;
+const PROXY_API_URL = API_ENDPOINTS.filters;
 
 /**
  * Filter metadata type from backend

src/shared/api/endpoints.ts

@@ -0,0 +1,19 @@
+/**
+ * Centralized backend endpoint definitions.
+ *
+ * One source of truth for the proxy API base URL — individual resource
+ * modules (proxyFonts, filters) append their own paths.
+ */
+
+export const API_BASE_URL = 'https://api.glyphdiff.com/api/v1' as const;
+
+export const API_ENDPOINTS = {
+    /**
+     * Font catalog + per-id detail + batch lookup
+     */
+    fonts: `${API_BASE_URL}/fonts`,
+    /**
+     * Filter metadata (providers, categories, subsets)
+     */
+    filters: `${API_BASE_URL}/filters`,
+} as const;

src/widgets/ComparisonView/model/stores/comparisonStore.svelte.ts

@@ -42,8 +42,10 @@ interface ComparisonState {
 
 export type Side = 'A' | 'B';
 
+const STORAGE_KEY = 'glyphdiff:comparison';
+
 // Persistent storage for selected comparison fonts
-const storage = createPersistentStore<ComparisonState>('glyphdiff:comparison', {
+const storage = createPersistentStore<ComparisonState>(STORAGE_KEY, {
     fontAId: null,
     fontBId: null,
 });

src/widgets/ComparisonView/ui/SliderArea/SliderArea.svelte

@@ -8,6 +8,11 @@
     - Performance optimized using offscreen canvas for measurements and transform-based animations.
 -->
 <script lang="ts">
+import {
+    MULTIPLIER_L,
+    MULTIPLIER_M,
+    MULTIPLIER_S,
+} from '$entities/Font';
 import { TypographyMenu } from '$features/AdjustTypography';
 import { typographySettingsStore } from '$features/AdjustTypography/model';
 import {
@@ -122,16 +127,16 @@ $effect(() => {
     }
     switch (true) {
         case responsive.isMobile:
-            typography.multiplier = 0.5;
+            typography.multiplier = MULTIPLIER_S;
             break;
         case responsive.isTablet:
-            typography.multiplier = 0.75;
+            typography.multiplier = MULTIPLIER_M;
             break;
         case responsive.isDesktop:
-            typography.multiplier = 1;
+            typography.multiplier = MULTIPLIER_L;
             break;
         default:
-            typography.multiplier = 1;
+            typography.multiplier = MULTIPLIER_L;
     }
 });
 

src/widgets/SampleList/model/stores/layoutStore/layoutStore.svelte.ts

@@ -28,7 +28,7 @@ interface LayoutConfig {
     mode: LayoutMode;
 }
 
-const STORAGE_KEY = 'glyphdiff:sample-list-layout';
+export const STORAGE_KEY = 'glyphdiff:sample-list-layout';
 const SM_GAP_PX = 16;
 const MD_GAP_PX = 24;
 

src/widgets/SampleList/model/stores/layoutStore/layoutStore.test.ts

@@ -16,8 +16,7 @@ async function flushEffects() {
     await Promise.resolve();
 }
 
-// Storage key used by LayoutManager
+import { STORAGE_KEY } from './layoutStore.svelte';
-const STORAGE_KEY = 'glyphdiff:sample-list-layout';
 
 describe('layoutStore', () => {
     // Default viewport for most tests (desktop large - >= 1536px)