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.
