[Feature]: Audition a search result before extracting it #442

Follow-on from #441.

The problem

Search returns a title, a channel and a duration. That is often not enough to tell a live take from a studio one, a remaster from the original, or the right cover from six wrong ones.

Getting it wrong costs a full separation, which is minutes. Hearing ten seconds would have answered it.

What it needs to do

• A control on each result that plays it, with pause and a seekable position bar
• Auditioning is not selecting. Pressing it must not choose the track or start an import.
• One at a time, and silent the moment the dropdown closes

The constraint that decides the design

The CSP allows media-src 'self' blob: data:. A googlevideo.com URL in an <audio> tag is blocked, and widening media-src so it would work lets any injected string in the webview pull media from anywhere. That CSP exists for #171 and should not be traded away for a convenience button.

So the audio has to be proxied: the browser talks to us, and we fetch a URL that yt-dlp resolved from an already-allowlisted page. The browser never talks to YouTube.

What that costs, and how to keep it small

• Resolving is the expensive half, roughly two seconds, and the result is stable for hours. It should be cached.
• The smallest audio-only stream is about 1.2 MB for a four minute track against 11.8 MB for the 360p video. For an audition, smallest wins.
• Range must be forwarded, so seeking is a partial request rather than a re-download.
• A paused <audio> keeps its connection open. For a proxied stream that means holding a socket on the backend for a preview nobody is listening to, so the src needs dropping on close, not just a pause.

Deliberately out of scope

Video. A progressive video+audio stream (format 18) is only reachable with the JS challenge solver, which is on hold in #438. It is also ten times the bytes for a question the audio already answers.

HLS. Refuse it rather than serve it and let it fail in the player. A manifest rewrite plus a segment proxy is a lot of machinery for an audition button.