refactor: share focus chrome and simplify the TV picker and browse paths

Focus chrome was implemented twice, once in the focusable wrapper and once
in the focus builders; both now go through FocusChrome. TvColorPicker's
channel row was a copy of TvNumberSpinner and is now that widget in compact
density.

Also trims unused helpers and fields and simplifies the Jellyfin browse
paths.

lib/providers/offline_watch_provider.dart

@@ -71,6 +71,7 @@ class OfflineWatchProvider extends ChangeNotifier with DisposableChangeNotifierM
   /// 2. Metadata from download provider
   ///
   /// Returns null if no position is available.
+  @visibleForTesting
   Future<int?> getViewOffset(String globalKey) async {
     // First check local offline progress
     final localOffset = await _syncService.getLocalViewOffset(globalKey);