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/player/player_provider.dart

@@ -104,7 +104,7 @@ class PlayerActions {
     await _ref.read(audioHandlerProvider).setVolume(v);
   }
 
-  /// Fetches /api/radio?seed_track=<id> and starts playing the
+  /// Fetches `/api/radio?seed_track=<id>` and starts playing the
   /// returned track list (seed at index 0 + recommended picks).
   Future<void> startRadio(String trackId) async {
     final dio = await _ref.read(dioProvider.future);