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/utils/live_tv_player_navigation_test.dart

@@ -8,10 +8,11 @@ import 'package:plezy/media/media_server_client.dart';
 import 'package:plezy/media/server_capabilities.dart';
 import 'package:plezy/providers/multi_server_provider.dart';
 import 'package:plezy/screens/video_player_screen.dart';
-import 'package:plezy/services/data_aggregation_service.dart';
 import 'package:plezy/services/multi_server_manager.dart';
 import 'package:plezy/utils/live_tv_player_navigation.dart';
 
+import '../test_helpers/multi_server_fixtures.dart';
+
 void main() {
   late MultiServerManager manager;
   late MultiServerProvider multiServer;
@@ -19,7 +20,7 @@ void main() {
   setUp(() async {
     await LocaleSettings.setLocale(AppLocale.bg);
     manager = MultiServerManager();
-    multiServer = MultiServerProvider(manager, DataAggregationService(manager));
+    multiServer = testMultiServerProvider(manager);
   });
 
   tearDown(() {