refactor(shared): rename fontCache to collectionCache

- Rename fontCache.ts to collectionCache.ts
- Rename FontCacheManager interface to CollectionCacheManager
- Make implementation fully generic (already was, just renamed interface)
- Update exports in shared/fetch/index.ts
- Fix getStats() to return derived store value for accurate statistics
- Add comprehensive test coverage for collection cache manager
  - 41 test cases covering all functionality
  - Tests for caching, deduplication, state tracking
  - Tests for statistics, reactivity, and edge cases

Closes task-1 of Phase 1 refactoring

package.json

@@ -61,5 +61,8 @@
     "vite": "^7.2.6",
     "vitest": "^4.0.16",
     "vitest-browser-svelte": "^2.0.1"
+  },
+  "dependencies": {
+    "@tanstack/svelte-query": "^6.0.14"
   }
 }