feat(player): complete MediaSession handlers + teardown

[EnesYilmazcode]

Fixes #10. Fixes #11.

Problem

• The lock screen / Control Center / CarPlay expose a stop control and 15s skip (seekbackward/seekforward) that had no handlers, so those affordances did nothing.
• The registration effect never cleared its handlers, so when the queue emptied the lock screen stayed bound to the previous track's stale closures.
• playbackState was set only from isPlaying, so it never reflected the "nothing loaded" state.

Fix

• Register stop, seekbackward, seekforward (plus existing seekto), each wrapped so an engine that doesn't support an action doesn't throw.
• Tear all handlers down on cleanup and when nothing is loaded (also clear metadata), via a shared MEDIA_ACTIONS list.
• Centralize playbackState as the single source of truth: 'none' when nothing is loaded, else mirror isPlaying — so iOS routes the next remote press to the correct action. Together with the reject-path sync from fix: reliable AirPods/lock-screen resume + correct the deploy base note #23, this closes [MEDIUM] MediaSession playbackState desyncs when audio.play() rejects (swallowed catch) #11.

Verification

Built clean; runtime smoke test:

Check	Result
Play → metadata.title set, playbackState	"Last Stop", playing ✅
Pause → playbackState	paused ✅
All handlers register (no throw)	none ✅

⚠️ The remote actions themselves (AirPods stem, lock-screen skip) can't be triggered in a headless browser — worth a quick real-device check, but registration + state sync are confirmed.

🤖 Generated with Claude Code

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Generated by Claude Code