Seafile sign-in follows the person across devices; sturdier Plex player; audiobooks frame with server-side secret sign-in

- Seafile token is saved on the server (encrypted) so Settings shows the real signed-in state everywhere
- Player: whole-track cache for current and next two, stall recovery, instant track changes, shuffle/repeat, Plex play counts, transcode fallback, Plex request timeout
- Audio tab: toolbar, full-height kept-alive frame; /api/audiobooks/enter adds the secret login link on the server

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

settings.js

@@ -8,9 +8,9 @@ const auth = require('./auth');
 const users = require('./users');
 const file = () => path.join(users.dir(), 'settings.json'); // each person has their own
 
-// Only these keys are accepted. (Per-device things like the Seafile sign-in token and the music queue stay in the browser.)
-const KEYS = new Set(['mail_hints', 'ab_creds', 'plex_lib', 'office_loc', 'cal_default', 'cal_view', 'cal_range', 'task_list', 'task_show_done', 'recents']);
-const SECRET = new Set(['ab_creds']);
+// Only these keys are accepted. (The music queue stays in the browser.)
+const KEYS = new Set(['mail_hints', 'ab_creds', 'sf_token', 'plex_lib', 'office_loc', 'cal_default', 'cal_view', 'cal_range', 'task_list', 'task_show_done', 'recents']);
+const SECRET = new Set(['ab_creds', 'sf_token']);
 const MAX_VALUE = 20000; // bytes of JSON per key
 
 let _key; const key = () => _key || (_key = crypto.scryptSync(auth.secret, 'nehub-settings', 32));