music: Music Player widget + app-wide player dock

The widget browses the /api/music/* library (search albums → tracklist) and hands
a queue to an app-wide player. The player (useMusicPlayer, useGlobal-backed) and
its site-wide bottom dock (MusicPlayerHost) live in the persistent DashboardLayout,
so playback survives route changes. Dock has cover/title/artist, drag-scrubbing
(SeekBar), volume (persisted), and prev/play/next/close. The nav Dock slides up by
MUSIC_DOCK_HEIGHT while the music dock is present.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/workspaces/widgets/package.json

@@ -8,6 +8,7 @@
     "./Pomodoro": "./Pomodoro/index.tsx",
     "./DailyGoals": "./DailyGoals/index.tsx",
     "./QuickNotes": "./QuickNotes/index.tsx",
-    "./Workspaces": "./Workspaces/index.tsx"
+    "./Workspaces": "./Workspaces/index.tsx",
+    "./MusicPlayer": "./MusicPlayer/index.tsx"
   }
 }