Improve appearance of transcript annotation UI for audio

The general video annotation functionality added in https://github.com/hypothesis/via/issues/1293 works if the "video" URL points to an audio file, since HTML `` elements can play audio-only files. However the UI looks odd because the space where the video content would normally appear is blank:

Other media players usually display some kind of placeholder or visualization in place of the video.

Start with the general video annotation functionality described in issue 1293 and trace how the media UI handles an audio file in an HTML video element. Update the audio-only presentation so the blank video area has a placeholder or visualization, and verify that transcript annotation still works with audio URLs.

Written by the indexing model from the issue text.