convert downloaded webm to mp4

[aigc-anime]

Checklist

• This feature I'm requesting is already implemented in yt-dlp.
• This feature is merely a UI/UX update.
• This feature is suitable for primary users with little knowledge about yt-dlp.
• This feature is available for most websites, not only the video platform I use.
• This feature is suitable for a large variety of videos.
• This feature is not going to conflict with many of the existing options.

Is your feature request related to a problem? Please describe.

I have some webm video and i would like to share them with someone who can only plan h264

Describe the solution you'd like

Add an option to convert downloaded webm to mp4 using ffmpeg on the phone.

Video link

No response

Additional context

No response

[JunkFood02]

Currently this is blocked by upstream which requires an update of ffmpeg, but you can download the h264 videos by setting the format preference to legacy for now

[NewNewPipe]

There is also a problem as of late with the latest version of Seal 1.12.0 also downloading comments.

Log:
[youtube] Extracting URL: https://www.youtube.com/watch?v=wDnY1B5uDSI
[youtube] wDnY1B5uDSI: Downloading webpage
[youtube] wDnY1B5uDSI: Downloading ios player API JSON
[youtube] wDnY1B5uDSI: Downloading android player API JSON
[youtube] wDnY1B5uDSI: Downloading m3u8 information
[youtube] Downloading comment section API JSON
[youtube] Downloading ~2306 comments
[youtube] Sorting comments by newest first
[youtube] Downloading comment API JSON page 1 (0/~2306)
[youtube] Downloading comment API JSON page 2 (0/~2306)
[youtube] Downloading comment API JSON page 3 (0/~2306)
[youtube] Downloading comment API JSON page 4 (0/~2306)
[youtube] Downloading comment API JSON page 5 (0/~2306)
[youtube] Downloading comment API JSON page 6 (0/~2306)
[youtube] Downloading comment API JSON page 7 (0/~2306)
[youtube] Downloading comment API JSON page 8 (0/~2306)
[youtube] Downloading comment API JSON page 9 (0/~2306)
[youtube] Downloading comment API JSON page 10 (0/~2306)
[youtube] Downloading comment API JSON page 11 (0/~2306)
[youtube] Downloading comment API JSON page 12 (0/~2306)
[youtube] Downloading comment API JSON page 13 (0/~2306)
[youtube] Downloading comment API JSON page 14 (0/~2306)
[youtube] Downloading comment API JSON page 15 (0/~2306)
[youtube] Downloading comment API JSON page 16 (0/~2306)
...

[aigc-anime]

would be even better if it can convert any external video (not only the ones downloaded by seal)