Wrap AudioPlayerScreen in audio_service for background playback (1b)

Creates PlayerAudioHandler (BaseAudioHandler + SeekHandler) that wraps
just_audio's AudioPlayer and bridges it to the Android media session:
background foreground-service playback, lock-screen / notification
controls (play/pause/seek/skip ±15 s), audio focus, and Bluetooth
headset events all handled by audio_service.

Key design decisions:
- Handler registered once via AudioService.init in main() and injected
  into ProviderScope via overrideWithValue (DIP: no global mutable var).
- Progress-sync timer stays in the screen so PlayerApiClient is never
  imported by the handler (SRP boundary preserved).
- Seek bar onChanged routes through handler.seek() so the notification
  position updates on slider drags (Law of Demeter fix).
- _initPlayer refactored into _buildAuthHeaders / _loadSource /
  _resumeFromSavedPosition helpers (each ≤30 lines, SoC).
- Tests override audioHandlerProvider with _FakePlayerAudioHandler to
  avoid platform-channel calls; all 221 tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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