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/src/services/audioService.ts

@@ -86,7 +86,7 @@ class AudioService {
     }
     await this.wavesurfer.play();
     if (songId && bandId) {
-      usePlayerStore.getState().setCurrentPlayingSong(songId, bandId);
+      usePlayerStore.getState().setCurrentSong(songId, bandId);
     }
   }
 
@@ -116,19 +116,10 @@ class AudioService {
     return this.isReady && !!this.wavesurfer;
   }
 
-  // Aliases kept for callers until Phase 3 cleans them up
-  public isReadyForPlayback(): boolean {
-    return this.isWaveformReady();
-  }
-
-  public canAttemptPlayback(): boolean {
-    return this.isWaveformReady();
-  }
-
   public cleanup(): void {
     this.destroyWaveSurfer();
     const store = usePlayerStore.getState();
-    store.setCurrentPlayingSong(null, null);
+    store.setCurrentSong(null, null);
     store.batchUpdate({ isPlaying: false, currentTime: 0 });
   }