Adds the three menu-driven player actions that back the TrackActions
overflow sheet. playNext/enqueue insert without disturbing playback
state; startRadio replaces the queue from GET /api/radio?seed_track=.
RadioController owns the API call so PlayerController stays Retrofit-free.
Endpoint is GET /api/radio?seed_track= (verified against
flutter_client/lib/api/endpoints/radio.dart), not the POST-with-path-
param shape originally drafted in the spec.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen
ac1832da43