feat(hub): Music app client — album grid, flat list, persistent player

Implements the client half of docs/musicbay.md against MNP 0.8:

- music-app.js: album grid (grouped by artist -> album, from index-time
  artist/album fields) or flat folder view, per-group localStorage
  toggle like Videos. MusicBrainz (music_meta_req) is only looked up
  when a track has no embedded cover at all — well-tagged files never
  trigger a network call, unlike Videos where TMDB is unconditional.
  Reuses video-app.js's MediaThumb/LazyTile (now exported) rather than
  duplicating the chunk-path thumbnail decode + virtualization.
- music-player.js: the persistent player bar — queue, shuffle (Fisher-
  Yates, keeps the current track in place when toggled), repeat (off/
  all/one), volume (localStorage), prev/next, a one-track prefetch
  cache. No MSE, no node-side streaming: a track is downloaded and
  decrypted once via file-utils.js's pipelinedDownload, same chunk
  pipeline Files already uses, then played from a blob URL.
- group-page.js: owns musicQueue/musicbrainzConfig state and renders
  MusicPlayerBar outside the tab-switched area — deliberately, so
  playback survives navigating to Chat/Files, the same reasoning the
  video/preview modals are shell-owned rather than app-owned.
- apps.js: registers "music". transport.js: fetchMusicMeta (keyed by
  path, same reordering-hazard fix as fetchMediaMeta),
  setMusicbrainzConfig/setMusicbrainzEnabled (signed ops, mirroring
  TMDB's), and the three new ack handlers. group-settings.js: a
  MusicBrainz settings section (contact string, per-group toggle) —
  the existing Applications checklist already picks up "music" for
  free, per apps.md's own claim.
- icon.js: music/pause/skip-next/skip-prev/shuffle/repeat/volume,
  drawn in the same stroked style as the existing set.
- i18n: group.tab_music, the music.* and settings_node.musicbrainz_*
  keys, translated (not just copied) across all ten locales, Polish
  carrying full one/few/many/other plural forms for music.n_tracks.
- webapp.py's _ASSETS, test_hook_ordering.py's STATIC_FILES and
  test_transport_contracts.py's SPLIT_FILES gain the two new files.

Full suite (common + hub + node): 1116 passed, no regressions.
`npm run sync-ui` in meshbay-client confirmed both files copied.

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

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