refactor(audio): Phase 4 — unify song tracking, remove compat aliases

playerStore: remove currentPlayingSongId/currentPlayingBandId/setCurrentPlayingSong.
Single pair (currentSongId/currentBandId) now set exclusively when play() is
called, not when the page opens. This means MiniPlayer and sidebar links only
appear after audio has been started — correct UX for a "now playing" widget.

audioService: play() calls setCurrentSong instead of setCurrentPlayingSong;
cleanup() clears it. Remove isReadyForPlayback() and canAttemptPlayback()
aliases — all callers now use isWaveformReady() directly.

useWaveform: remove setCurrentSong call from init (store updated by play()
now); restore-playback snapshot reads currentSongId/currentBandId.

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

web/tests/audioService.test.ts

@@ -164,19 +164,19 @@ describe('AudioService', () => {
       expect(mockWs.play).toHaveBeenCalled();
     });
 
-    it('updates currentPlayingSongId/BandId in the store', async () => {
+    it('updates currentSongId/BandId in the store', async () => {
       await initService(service);
       await service.play('song-1', 'band-1');
       const state = usePlayerStore.getState();
-      expect(state.currentPlayingSongId).toBe('song-1');
-      expect(state.currentPlayingBandId).toBe('band-1');
+      expect(state.currentSongId).toBe('song-1');
+      expect(state.currentBandId).toBe('band-1');
     });
 
     it('does not update store ids when called without ids', async () => {
       await initService(service);
       await service.play();
       const state = usePlayerStore.getState();
-      expect(state.currentPlayingSongId).toBeNull();
+      expect(state.currentSongId).toBeNull();
     });
   });