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/hooks/useWaveform.ts

@@ -41,17 +41,12 @@ export function useWaveform(
       try {
         await audioService.initialize(containerRef.current!, options.url!);
 
-        // Update global song context
-        if (options.songId && options.bandId) {
-          usePlayerStore.getState().setCurrentSong(options.songId, options.bandId);
-        }
-
         // Restore playback if this song was already playing when the page loaded.
         // Read as a one-time snapshot — these values must NOT be reactive deps or
         // the effect would re-run on every time update (re-initializing WaveSurfer).
         const {
-          currentPlayingSongId,
-          currentPlayingBandId,
+          currentSongId,
+          currentBandId,
           isPlaying: wasPlaying,
           currentTime: savedTime,
         } = usePlayerStore.getState();
@@ -59,8 +54,8 @@ export function useWaveform(
         if (
           options.songId &&
           options.bandId &&
-          currentPlayingSongId === options.songId &&
-          currentPlayingBandId === options.bandId &&
+          currentSongId === options.songId &&
+          currentBandId === options.bandId &&
           wasPlaying &&
           audioService.isWaveformReady()
         ) {