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