Gapless playback

[jcraigk]

Allow for gapless playback on the web-based player.

#333 switched from SoundManager2 to native web audio. We use a single audio element and switch the src on it for each track. We should investigate ways to preload audio and/or use multiple audio nodes.

Investigating this with ChatGPT, looks like we need to use an audio node for each track, scrub, and play/pause. Also the two different APIs each have their limitations. We could look at https://www.npmjs.com/package/@regosen/gapless-5/v/0.9.3 or something similar, but probably tricky to implement.

Another idea is to combine all tracks for a show into a single audio file, attach that to the show, and add "starts_at_millisecond" on tracks so the player can keep the state matched with the audio. We could continue to fallback to individual mp3s for download and playing in playlists.