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/api/endpoints/radio.dart

@@ -2,11 +2,11 @@ import 'package:dio/dio.dart';
 
 import '../../models/track.dart';
 
-/// GET /api/radio?seed_track=<uuid>&limit=<int>. Returns the seed at
-/// index 0 followed by up to `limit-1` weighted-shuffle picks scored
-/// by the server's recommendation engine. The shape matches what
-/// `playerActions.playTracks` expects, so a radio start is just one
-/// fetch + one playTracks call.
+/// `GET /api/radio?seed_track=<uuid>&limit=<int>`. Returns the seed
+/// at index 0 followed by up to `limit-1` weighted-shuffle picks
+/// scored by the server's recommendation engine. The shape matches
+/// what `playerActions.playTracks` expects, so a radio start is just
+/// one fetch + one playTracks call.
 class RadioApi {
   RadioApi(this._dio);
   final Dio _dio;