Downloads bypass stream selection and reach play() as ready-made local
file streams, so audio mode never affected them. Apply the mode at that
choke point instead: swap a local video stream for the separately
downloaded audio track, or play a muxed file with the video track
disabled (vid set per-load in MPVClient, never toggled live). The
transform is bidirectional so stored audio-only queue/history entries
restore full video when the mode is off, and setAudioMode now reloads
downloaded content at the current position in both directions.
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