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/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
-const STORAGE_KEY = 'glyphdiff:sample-list-layout';
+import { STORAGE_KEY } from './layoutStore.svelte';
 
 describe('layoutStore', () => {
     // Default viewport for most tests (desktop large - >= 1536px)