[feature/frontend] Add player for audio files; use thumbnail for poster

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

In #3090 support was added for audio files, but no web frontend stuff was in place for it yet.

This PR:

• adds audio support to the web frontend so that audio files can be played just like video files, using our existing player
• adds a new status from zork to the testrig, in which he posts an audio file (the included audio file is Creative Commons licensed and is attributed in the media description).
• updates some other frontend player stuff to use video poster instead of the video file itself as the video thumbnail. This should also improve page loading performance on pages with lots of videos
• takes account of some Mastodon API eccentricities by providing framerate and original dimensions for audio files
• collapses media gallery into single column when viewing on skinny screens, so the media controls don't become unusable

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

nice work! thanks for getting the frontend side of this working 😎