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>

.env.example

@@ -13,3 +13,9 @@ VERSE_URL=https://versebyversewithnate.us/admin
 PODCAST_URL=https://podcast.necloud.us/
 # Days you stay signed in without using the app (it renews every time you do)
 SESSION_DAYS=180
+
+# Audiobooks secret sign-in link (optional; anyone holding it is signed in there, so never commit it)
+AUDIOBOOKS_LOGIN_LINK=
+# or per-person signed links (optional): shared secret the audiobooks site uses to verify signed links
+AUDIOBOOKS_SSO_SECRET=
+#AUDIOBOOKS_SSO_URL=https://audiobooks.necloud.us/sso