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/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
-const STORAGE_KEY = 'glyphdiff:theme';
+import { STORAGE_KEY } from './ThemeManager.svelte';
 
 // Helper type for MediaQueryList event handler
 type MediaQueryListCallback = (this: MediaQueryList, ev: MediaQueryListEvent) => void;