fix(flutter): CI analyze + center title in full player

- artist_detail_screen.dart missed an `import '../models/artist.dart'`
  for the ArtistRef seed parameter; analyze flagged undefined_class.
- radio.dart + player_provider.dart doc comments wrapped URL
  parameters in <...> which lint reads as HTML. Switched to backticks.

Full player title is now centered absolutely via a Stack: title with
horizontal padding equal to the actions cluster width sits at the
optical center, while LikeButton + TrackActionsButton are pinned to
the right edge with Positioned. Fixed-height SizedBox(32) keeps the
row stable when the title wraps to a single ellipsized line.

flutter_client/lib/library/artist_detail_screen.dart

@@ -5,6 +5,7 @@ import 'package:go_router/go_router.dart';
 import '../api/endpoints/likes.dart';
 import '../likes/like_button.dart';
 import '../models/album.dart';
+import '../models/artist.dart';
 import '../player/player_provider.dart';
 import '../shared/widgets/server_image.dart';
 import '../theme/theme_extension.dart';