feat: add persistent cover image cache

- Add CoverCacheManager service for persistent image caching
- Cache stored in app_flutter/cover_cache/ (not cleared by system)
- Maximum 1000 images cached for up to 365 days
- Update all 11 screens to use persistent cache manager
- Add flutter_cache_manager and path dependencies
- Update CHANGELOG.md with all changes for v3.1.3

pubspec.lock

@@ -327,7 +327,7 @@ packages:
     source: sdk
     version: "0.0.0"
   flutter_cache_manager:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: flutter_cache_manager
       sha256: "400b6592f16a4409a7f2bb929a9a7e38c72cceb8ffb99ee57bbf2cb2cecf8386"
@@ -662,7 +662,7 @@ packages:
     source: hosted
     version: "0.3.3+7"
   path:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: path
       sha256: "75cca69d1490965be98c73ceaea117e8a04dd21217b37b292c9ddbec0d955bc5"