Export edited media as video (MP4/WebM) via mediabunny

[maboa]

Background

For video sources, the current export drops the video track entirely — the output is a mono WAV. That makes the redaction and gap-removal features much less useful on video content. Mediabunny can read and write real video containers (MP4, WebM, MKV) via WebCodecs, so we can produce a video file with the cuts applied.

Proposed change

When the source media has a video track, the export modal exposes video output options:

• MP4 (H.264/AAC, broad compatibility).
• WebM (VP9/Opus, smaller, open).

The export pipeline:

1. Read source frames (audio + video) via mediabunny's Input.
2. For each section in audioDataArray, copy frames whose timestamps fall within the section.
3. Write to a mediabunny Output in the chosen container/codec.
4. Audio and video tracks remain in sync.

Acceptance criteria

• Modal detects whether the source has a video track and conditionally shows video format options.
• Exported video is a valid file, plays in browsers and standard players.
• Audio and video stay aligned after cuts.
• Section boundaries are honored — either by aligning cuts to keyframes, or by re-encoding the GOP around the cut.

Open questions / risks

• Pure remux (no re-encode) is fast but only frame-accurate at keyframes. Re-encoding the boundary GOP gives accurate cuts but is slower and may lose quality. Default behavior needs to be decided.
• Some codec/container combinations may not be supported by every browser. Need to pick safe defaults and surface limitations.

Out of scope

• Bitrate / codec advanced controls (separate issue).