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/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,
 });