Save settings on the server and sync them to every device

Preferences lived only in the browser, so clearing it, using another device or the separate iOS
home-screen storage lost them. They are now saved in /data/settings.json (the audiobooks sign-in
encrypted), loaded at start-up, and existing browser-only values are uploaded once. A failed save
is reported instead of silent.

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

README.md

@@ -34,6 +34,7 @@ e.g. hub.necloud.us. Settings: `SEAFILE_URL`, `ONLYOFFICE_URL` env vars in docke
 - **Tasks**: to-do lists stored in NE Hub (`/data/tasks.json`), with due dates, notes and completion. If the connected iCloud
   account exposes reminder lists over CalDAV they appear too and sync both ways. Apple hides Reminders lists that were upgraded
   for iOS 13+, so for most accounts only NE Hub lists are available.
+- **Settings sync**: your preferences (mail addresses, audiobooks sign-in (encrypted), Plex library, Office save location, calendar/task views, recent files) are saved on the server in `/data/settings.json` and shared by every device. Per-device items (Seafile sign-in token, music queue) stay in the browser.
 - **Search**: libraries, file names (scans libraries if Seafile has no search index) and Plex artists/albums/tracks.
 - **Music**: Plex player. Sign in goes through plex.tv (no password handled), the server proxies your Plex server and adds the token itself, and a mini player keeps playing as you move between tabs; the queue (play next / add to queue) and position are remembered.
 - **Audio**: embeds audiobooks.necloud.us; sign-in details saved in Settings can be copied with one tap.