feat: add local library albums to Albums tab with unified grid and LocalAlbumScreen

- Add local library albums as clickable cards in Albums filter
- Merge downloaded and local albums into single unified grid (fix layout gaps)
- Create LocalAlbumScreen for viewing local album details with:
  - Cover art display with dominant color extraction
  - Album info card with Local badge and quality info
  - Track list with disc grouping support
  - Selection mode with delete functionality
  - UI consistent with DownloadedAlbumScreen (Card + ListTile layout)
- Add singles filter support for local library singles
- Add extractDominantColorFromFile to PaletteService
- Add delete(id) method to LibraryDatabase
- Add removeItem(id) method to LocalLibraryNotifier
- Update CHANGELOG.md for v3.4.0

ios/Runner/AppDelegate.swift

@@ -688,6 +688,12 @@ import Gobackend  // Import Go framework
             return response
             
         // Local Library Scanning
+        case "setLibraryCoverCacheDir":
+            let args = call.arguments as! [String: Any]
+            let cacheDir = args["cache_dir"] as! String
+            GobackendSetLibraryCoverCacheDirJSON(cacheDir)
+            return nil
+            
         case "scanLibraryFolder":
             let args = call.arguments as! [String: Any]
             let folderPath = args["folder_path"] as! String