Cover block: show link to embedded video after embedding

[annezazu]

Thanks to the ability to add external media to the cover block in #73023, you can now embed videos from sources like YouTube. However, if you do this and then want to see what video is added externally, it's not possible:

cover.block.mov

In my mind, when someone selects "Embed video from URL" when one has already been set, we should show that link and folks can overwrite it, similar to how this works now with the media library option:

Screen.Recording.2026-01-22.at.7.50.22.PM.mov

When you select media library, it shows upon opening the current image that's selected/shown in the Cover block.

[Mustafabharmal]

I am working on these.