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>

docker-compose.yml

@@ -15,6 +15,9 @@ services:
       SEAFILE_URL: ${SEAFILE_URL:-https://seafile.necloud.us}
       ONLYOFFICE_URL: ${ONLYOFFICE_URL:-https://onlyoffice.necloud.us}
       AUDIOBOOKS_URL: ${AUDIOBOOKS_URL:-https://audiobooks.necloud.us}
+      AUDIOBOOKS_LOGIN_LINK: ${AUDIOBOOKS_LOGIN_LINK:-}
+      AUDIOBOOKS_SSO_SECRET: ${AUDIOBOOKS_SSO_SECRET:-}
+      AUDIOBOOKS_SSO_URL: ${AUDIOBOOKS_SSO_URL:-}
       VERSE_URL: ${VERSE_URL:-https://versebyversewithnate.us/admin}
       PODCAST_URL: ${PODCAST_URL:-https://podcast.necloud.us/}
       ONLYOFFICE_JWT_SECRET: ${ONLYOFFICE_JWT_SECRET:-}    # same value as the OnlyOffice server's JWT secret