refactor: share the paginated grid tab, cached remote store, and tile focus

- PaginatedCardGridTabState: the collections and playlists tabs were 95%
  identical; they now supply only pageSize/fetchPage/idOf instead of each
  duplicating the grid, memo, inflation budget and focus wiring.
- EtagCachedRemoteStore: the anime-lists and fribb mapping stores now share
  one download/cache/isolate-parse/conditional-GET lifecycle.
- FocusableTileStateMixin manages its own initState/didUpdateWidget/dispose
  instead of requiring every caller to forward three lifecycle hooks.

Also drops unused ServerCapabilities entries and dead code in
focusable_list_tile and music/track_row.

test/navigation/profile_session_screen_test.dart

@@ -16,13 +16,13 @@ import 'package:plezy/providers/discover_provider.dart';
 import 'package:plezy/providers/hidden_libraries_provider.dart';
 import 'package:plezy/providers/multi_server_provider.dart';
 import 'package:plezy/providers/trackers_provider.dart';
-import 'package:plezy/services/data_aggregation_service.dart';
 import 'package:plezy/services/multi_server_manager.dart';
 import 'package:plezy/services/offline_watch_sync_service.dart';
 import 'package:plezy/services/storage_service.dart';
 import 'package:plezy/services/system_shelf_service.dart';
 import 'package:provider/provider.dart';
 
+import '../test_helpers/multi_server_fixtures.dart';
 import '../test_helpers/prefs.dart';
 
 void main() {
@@ -51,7 +51,7 @@ void main() {
       storage: storage,
     );
     final serverManager = MultiServerManager();
-    final multiServer = MultiServerProvider(serverManager, DataAggregationService(serverManager));
+    final multiServer = testMultiServerProvider(serverManager);
     // The session tree instantiates MusicPlaybackServiceImpl (the mini-player
     // overlay watches it), which needs the database + offline watch service.
     final offlineWatch = OfflineWatchSyncService(database: db, serverManager: serverManager);