Add a shared scrubber to each good/bad video pair

Native <video controls> would give each clip its own timeline — and the
two clips in a pair differ by 1-3 s, so they drift apart — while the
control bar would also cover the on-screen WASD overlay. Instead each
pair gets one custom control row:

- play/pause and seek drive both clips together
- looping is handled in JS so the pair restarts in step
- ticks mark the command switches (20 s, 40 s) on the track
- explicit pause is remembered, so scrolling back does not resume

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: xjxu21