feat(hub): play audio files from Explorer, scoped to the current folder

Audio files were invisible to file-utils.js's canPreview, so their
name never rendered as a clickable link and the toolbar's Play/View
buttons never enabled for them - the only way to do anything with an
audio file in Files was to download it.

- file-utils.js: canPreview now includes 'audio' alongside image/
  video/document.
- files-app.js: the toolbar's canPlay/canView split treats audio like
  video (Play, not View) rather than falling into the generic preview
  path, which was never built for it anyway.
- group-page.js: onPreview routes an audio entry to onPlayQueue - the
  same persistent player Music uses - instead of the preview modal.
  Deliberately not Music's artist/album grouping: the queue is every
  audio entry sharing the clicked file's literal containing directory
  (entry.path), sorted by filename, so previous/next in Explorer stays
  scoped to what's actually in that folder, tags or no tags. onPlayQueue
  already replaces whatever queue is playing unconditionally, so
  starting a track from Files while Music (or another Files folder) is
  already playing needs no special handling - it's the same "just a new
  queue" path either way.

Client-side only, no protocol/index change. npm run sync-ui re-run.
Full suite: 1129 passed, no regressions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KBi7ALLGfwcjBXt57yNMcy

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