Recording now uses the browser's hardware video encoder:
- JPEG frames drawn to offscreen canvas → canvas.captureStream(30fps)
- MediaRecorder encodes to MP4 (preferred) or WebM (fallback)
- Stop triggers download of standard video file
Playback uses native <video> element — no custom parsing needed.
Massively simplified: removed packFrames/unpackFrames, no more custom
binary format.
