Audio-clocked playback for judging sync

Lip sync cannot be judged silently. extract.sh now pulls the audio track and
writes manifest.json alongside the frames; the page reads the true extraction
rate from it rather than assuming one, since a guessed fps desynchronises
picture from sound - the one thing this view exists to show.

Audio is the clock: frame = floor(currentTime * fps). A slow render loop drops
frames instead of drifting, and half/quarter speed work via playbackRate with
the picture following for free. Scrubbing, stepping and clicking a thumbnail
all seek the audio too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

manifest.json

@@ -0,0 +1 @@
+{"fps":12,"frames":37,"dir":"frames","audio":"audio.wav","source":"IMG_8486.MOV"}