feat(output): add thumbnail format setting so embedded cover art is readable

[Phr33d0m]

Why

I download YouTube videos as MKV with "Add cover image" enabled, but Dolphin (KDE Plasma) never showed the YouTube thumbnail as the file preview, just a random frame from the video.

The cover is embedded fine. It is embedded as cover.webp, and FFmpeg only treats an attachment as cover art when its mime type is jpeg/png/gif/tiff. I embedded the same image as webp, jpeg and png into otherwise identical files: only jpeg and png produced a thumbnail.

Nothing in the app could change this, since we let yt-dlp attach whatever the site served.

What this adds

A Thumbnail format setting (JPEG / PNG / Keep original) that passes --convert-thumbnails to yt-dlp.

It also changes the extension of files saved by "Save thumbnail file", because yt-dlp converts every thumbnail on disk, not just the embedded one. The hint text mentions this.

MP4 benefits too. yt-dlp converts webp to PNG there, which is much larger than JPEG and unreliable in Windows Explorer.

Default

Defaults to JPEG, so MKV covers start working and saved thumbnails become .jpg instead of .webp.

"Add cover image" is already on by default and currently produces a cover most software cannot read, so I treated this as a fix rather than a new preference. It is still a behaviour change though, so if you would rather default to "Keep original", say so and I will flip it.

Testing

cargo test 128 passed, npm run test:unit 131 passed, build/clippy/eslint clean.

Real download vs a control on current behaviour:

$ mkvmerge -J out.mkv          # setting on JPEG
  {'content_type': 'image/jpeg', 'file_name': 'cover.jpg', ...}
$ mkvmerge -J control.mkv      # current behaviour
  {'content_type': 'image/webp', 'file_name': 'cover.webp', ...}

Adds 7 backend and 4 frontend tests, updates 7 existing argument tests for the new default. English locale only, per CONTRIBUTING.

Screenshots

Disclosure

This PR and code have been AI generated.