Music: Featured tab with a random artist and album, from the whole library or chosen playlists

- Featured is the first Music tab; picks stay for the day on a device, Pick again rolls new ones
- Based on: choose the playlists they come from (synced across devices)
- Play mix shuffles the artist's songs; Play plays the album

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/sw.js

@@ -1,5 +1,5 @@
 // App-shell cache only; Seafile/API traffic always goes to the network.
-const CACHE = 'hub-v55';
+const CACHE = 'hub-v56';
 const SHARE = 'hub-share'; // files shared into the app from other apps, held until the person picks a folder
 const SHELL = ['/icon.svg', '/manifest.webmanifest', '/icons/icon-192.png', '/icons/apple-touch-icon.png'];