refactor: migrate local library from in-memory list to database-backed pagination

Replace the full in-memory List<LocalLibraryItem> in LocalLibraryState
with a lightweight lookup index (ISRCs, matchKeys, filePathById) and
database-backed FutureProvider.family pagination providers.

Database changes:
- Add library schema v7 with normalized lookup columns (track_name_norm,
  artist_name_norm, album_name_norm, album_artist_norm, match_key,
  album_key) and corresponding indexes
- Backfill normalized columns on migration from v6
- Add getPage, getPageCount, getAlbumPage, getAlbumCount, getLookupIndex,
  getCoverPaths, getByFilePath, findFirstByTrackAndArtist DB methods

Provider changes:
- LocalLibraryState no longer holds items list; uses totalCount and
  loadedIndexVersion for change tracking
- Deprecate synchronous getByIsrc/findByTrackAndArtist (return null);
  add async findExistingAsync, getByIsrcAsync, getById on notifier
- Add localLibraryPageProvider, localLibraryAlbumPageProvider,
  localLibraryAllItemsProvider family providers for paginated access
- Add localLibraryCoverProvider and localLibraryFirstCoverProvider
  for async cover path resolution from DB

Screen migrations:
- album/artist/playlist screens use findExistingAsync for playback
- library_tracks_folder_screen uses async cover providers and
  existsInLibrary for local library indicator
- queue_tab watches localLibraryAllItemsProvider instead of state.items
- library_settings_page uses state.totalCount
- playback_provider uses findExistingAsync

Track metadata screen:
- Replace pushReplacement navigation with in-place state swap using
  AnimatedSwitcher for smooth cross-fade transitions on track swipe
- Add metadataLoadGeneration counter to prevent stale async callbacks
- Reset all transient state (lyrics, cover, file check) on track change

lib/screens/queue_tab.dart

@@ -2369,7 +2369,12 @@ class _QueueTabState extends ConsumerState<QueueTab> {
       settingsProvider.select((s) => s.localLibraryEnabled),
     );
     final localLibraryItems = localLibraryEnabled
-        ? ref.watch(localLibraryProvider.select((s) => s.items))
+        ? ref
+              .watch(localLibraryAllItemsProvider)
+              .maybeWhen(
+                data: (items) => items,
+                orElse: () => const <LocalLibraryItem>[],
+              )
         : const <LocalLibraryItem>[];
     // Watch with selector on key fields to reduce unnecessary rebuilds.
     // LibraryCollectionsState doesn't implement == so watching without