Handle NAS deploy without useradd

Prompts used since the previous commit:
- sudo: useradd: command not found

Special observations:
- If useradd is unavailable, remote deployment now falls back to the existing SSH user and primary group.
- The systemd template now has a separate group placeholder.
- verplaats_log.json remains modified from the user's run and was intentionally not staged.

systemd/mailcat-sort.service

@@ -6,7 +6,7 @@ Wants=network-online.target
 [Service]
 Type=simple
 User=__APP_USER__
-Group=__APP_USER__
+Group=__APP_GROUP__
 WorkingDirectory=__APP_DIR__
 ExecStart=/usr/bin/python3 __APP_DIR__/sort_mail_daemon.py --account __ACCOUNT__ --folder INBOX --state-file /var/lib/mailcat/sort_mail_daemon_state.json --log-file /var/log/mailcat/sort_mail_daemon.log
 Restart=always