fix(flutter): player updates on track change + kebab artist nav + Start radio

Three issues, all related to the player surface:

1. Player UI didn't update on track change. audio_handler's
   _onCurrentIndexChanged only kicked off the cover load — it never
   pushed the new MediaItem onto the mediaItem stream. Title/artist/
   cover stayed pinned to whatever setQueueFromTracks(initialIndex:)
   set on first play. Now the listener pushes queue[idx] when the
   index changes.

2. Player kebab "Go to artist" 404'd while the same item from
   MostPlayed worked. Same TrackActionsSheet for both, but the
   player's _trackRefFromMediaItem was hardcoding artistId: ''
   because audio_handler's _toMediaItem never stashed it in extras.
   Stash artist_id alongside album_id; player_bar +
   now_playing_screen read it back. Both kebabs now navigate.

3. "Start radio" didn't exist on Flutter even though the server has
   /api/radio?seed_track=<id>. New RadioApi (lib/api/endpoints/
   radio.dart) wraps the endpoint; PlayerActions.startRadio(trackId)
   fetches + plays the result via the existing playTracks path.
   New menu item between "Add to playlist" and the divider above
   "Go to album", calls startRadio with a snackbar error fallback.

flutter_client/lib/player/now_playing_screen.dart

@@ -235,7 +235,7 @@ class _TitleRow extends StatelessWidget {
           title: media.title,
           albumId: (media.extras?['album_id'] as String?) ?? '',
           albumTitle: media.album ?? '',
-          artistId: '',
+          artistId: (media.extras?['artist_id'] as String?) ?? '',
           artistName: media.artist ?? '',
           durationSec: media.duration?.inSeconds ?? 0,
           streamUrl: '',