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>
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