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>

settings.js

@@ -9,7 +9,7 @@ const users = require('./users');
 const file = () => path.join(users.dir(), 'settings.json'); // each person has their own
 
 // Only these keys are accepted. (The music queue stays in the browser.)
-const KEYS = new Set(['ab_creds', 'sf_token', 'plex_lib', 'office_loc', 'cal_default', 'cal_view', 'cal_range', 'task_list', 'task_show_done', 'recents', 'links', 'office_pins', 'nav_pins']);
+const KEYS = new Set(['ab_creds', 'sf_token', 'plex_lib', 'music_feat_pls', 'office_loc', 'cal_default', 'cal_view', 'cal_range', 'task_list', 'task_show_done', 'recents', 'links', 'office_pins', 'nav_pins']);
 const SECRET = new Set(['ab_creds', 'sf_token']);
 const MAX_VALUE = 20000; // bytes of JSON per key