refactor(comparison): replace comparisonStore singleton with lazy getComparisonStore

Mirror the font-catalog change in ComparisonView: expose getComparisonStore()
(plus __resetComparisonStore for tests) instead of an eager comparisonStore
singleton, and consume getFontCatalog() internally. Update the model barrel and
all UI consumers (Sidebar, FontList, Header, Line, SliderArea); Character no
longer needs the store and reads everything from props.

Update both specs to the accessor: comparisonStore.test mocks getFontCatalog
with a writable stub (the real store's fonts is getter-only) and resets the
catalog between cases; Sidebar.svelte.test resolves the store via the accessor.

Also document Character's props.

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

@@ -19,9 +19,10 @@ import {
     getFontUrl,
 } from '$entities/Font';
 import {
+    type FontCatalogStore,
     FontsByIdsStore,
-    fontCatalogStore,
     fontLifecycleManager,
+    getFontCatalog,
 } from '$entities/Font/model';
 import { typographySettingsStore } from '$features/AdjustTypography/model';
 import { createPersistentStore } from '$shared/lib';
@@ -98,10 +99,13 @@ export class ComparisonStore {
      */
     #fontsByIdsStore: FontsByIdsStore;
 
+    #fontCatalog: FontCatalogStore;
+
     constructor() {
         // Synchronously seed the batch store with any IDs already in storage
         const { fontAId, fontBId } = storage.value;
         this.#fontsByIdsStore = new FontsByIdsStore(fontAId && fontBId ? [fontAId, fontBId] : []);
+        this.#fontCatalog = getFontCatalog();
 
         $effect.root(() => {
             // Sync batch results → fontA / fontB
@@ -173,7 +177,7 @@ export class ComparisonStore {
                     return;
                 }
 
-                const fonts = fontCatalogStore.fonts;
+                const fonts = this.#fontCatalog.fonts;
 
                 if (fonts.length < 2) {
                     return;
@@ -356,7 +360,14 @@ export class ComparisonStore {
     }
 }
 
-/**
- * Singleton comparison store instance
- */
-export const comparisonStore = new ComparisonStore();
+let _comparisonStore: ComparisonStore | undefined;
+
+export function getComparisonStore(): ComparisonStore {
+    return (_comparisonStore ??= new ComparisonStore());
+}
+
+// test-only reset, so specs don't share a live observer
+export function __resetComparisonStore() {
+    _comparisonStore?.resetAll();
+    _comparisonStore = undefined;
+}