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/services/plex_client.dart

@@ -915,6 +915,7 @@ class PlexClient
     );
   }
 
+  @visibleForTesting
   Future<Map<String, dynamic>> getServerIdentity() async {
     final response = await _getWithFailover('/identity');
     return response.data;