Add database backup/restore feature with admin UI

- Backup service using VACUUM INTO for WAL-safe atomic snapshots
- Server-side storage in DATA_DIR/backups with download/upload API routes
- Restore with integrity validation and automatic pre-restore safety backup
- Scheduled daily backups via cron with configurable retention
- Admin-only settings UI consolidated under single Server section
- Clean up account section sign-out button, fix switch sub-pixel rendering

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/globals.css

@@ -100,7 +100,8 @@
 
   /* https://ui.shadcn.com/docs/components/radix/button#cursor */
   button:not(:disabled),
-  [role="button"]:not(:disabled) {
+  [role="button"]:not(:disabled),
+  [role="switch"]:not(:disabled) {
     cursor: pointer;
   }
 }