Restart video from the beginning on reveal

[modem7]

Summary

• The video autoplays muted from t=0 the moment the page loads, but the decoy overlay (loading/error) can hold someone's attention for anywhere from a second to several minutes before they actually click it. By the time they do, the unmuted playback lands wherever the video has already drifted to - often well past the intro.
• Seeks video.currentTime = 0 right before unmuting/playing in reveal(), so the payoff always starts from the actual song opening regardless of how long the click took.
• Always-on behavior, no new env var - there's no real scenario where landing mid-song instead of on the intro is the desired outcome.

Buffering is unaffected by this change. The video still starts fetching/buffering the instant the page loads, exactly as before - it's still autoplaying muted the whole time the decoy is up, so playback is never "starting fresh" on click. This change only rewinds the playback position at the moment of reveal; it doesn't change when or how the browser loads the video. Since the start of the file is therefore already buffered (that's the part that's been playing muted since page load), seeking back to it on click is instant with no rebuffering or stutter.

Fixes #121

Also in this PR

• Playwright's browser binaries are now cached across CI runs (pinned to an exact version so the cache key can't silently go stale), and the Docker build's video-fetch stage is cached via GHA cache the same way ghcr-publish.yml already does. Cuts a cold CI run from ~3min to ~1m30s.
• test-autoplay.js now lets muted autoplay run for a few seconds before clicking, then asserts currentTime resets to near 0 after - regression coverage for the restart-on-reveal fix above.

Test plan

• CI (test-autoplay.js, both overlays, Chromium + Firefox, including the new currentTime assertion) and test-mobile-fit.js all passing.
• Verified on a second CI run that both new caches (Playwright browsers, Docker video-fetch layer) actually hit and skip re-downloading.
• Manual check that clicking after a deliberate delay still starts the audio from the beginning.