feat(hub): audio_root wiring, mutually-exclusive players, Settings rework

Five related pieces of polish against the Music app and Settings, all
from the same conversation:

- Music app now requires audio_root, same as Videos requires video_root:
  an empty-state message until one is set, and grouping filtered to only
  what's under it (underAudioRoot, mirroring video-app.js's
  underVideoRoot). Wires the new audio_root/audio_root_ack pair through
  transport.js and group-page.js state the same way video_root already
  flows.
- Starting one player now stops the other — opening a film closes the
  music queue, starting a track closes the video modal. Both used to run
  at once, found live.
- Group Settings reworked: every section but a bare form (invite,
  pair-operator, approve-device) is now collapsible (CollapsibleSection);
  the uploads on/off button is a real toggle switch (ToggleSwitch,
  reused for TMDB/MusicBrainz's enabled switches too, each now with an
  icon + status badge in its header instead of a plain checkbox row);
  and shared directories, the Videos root picker, and the new Music root
  picker are merged into one "Directories" section (RootFolderRow) instead
  of three separate ones scattered down the page — the root pickers only
  show once their app is actually enabled.

1 files changed, 9 insertions, 0 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
index db7d048..1101eb2 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
@@ -160,6 +160,7 @@ export default {
   'video.mode_flat': 'Flat list',
   'video.empty': 'No videos found.',
   'video.no_root_configured': 'No Videos root folder is set for this group yet — an operator can choose one in Settings.',
+  'music.no_root_configured': 'No Music root folder is set for this group yet — an operator can choose one in Settings.',
   'video.director': 'Director',
   'video.specials': 'Specials',
   'video.season_n': 'Season {n}',
@@ -470,6 +471,14 @@ export default {
   'settings_node.video_root_save': 'Save',
   'settings_node.video_root_change_confirm': 'Changing the Videos root replaces what every member sees in the Videos tab. Continue?',
 
+  'settings_node.audio_root_title': 'Music root folder',
+  'settings_node.audio_root_hint': 'Which folder (or subfolder) the Music app treats as its entry point for this group. Nothing shows in Music, and no tag/cover reading runs, until one is chosen.',
+  'settings_node.audio_root_none': '— none chosen —',
+  'settings_node.audio_root_save': 'Save',
+  'settings_node.audio_root_change_confirm': 'Changing the Music root replaces what every member sees in the Music tab. Continue?',
+  'settings_node.directories_title': 'Directories',
+  'settings_node.directories_hint': 'Shared folders, and which of them the Videos and Music apps use as their own entry point.',
+
   // Members
   'members.col_role': 'Role',
   'members.group_role': 'Group role',