Add QNAP plain deployment mode

Prompts used since the previous commit:
- Doe maar
- De homedirectory voor mailcat moet zijn /share/homes/mailcat

Special observations:
- Added SERVICE_MANAGER=plain for QNAP-style NAS hosts without systemd.
- Plain mode installs start/stop/status scripts under /share/homes/mailcat/mailcat/bin and uses /opt/bin/python3 by default.
- Remote install now uses explicit absolute tool path variables supplied for the NAS.
- verplaats_log.json remains modified from the user's run and was intentionally not staged.

systemd/mailcat-sort.service

@@ -8,14 +8,14 @@ Type=simple
 User=__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
+ExecStart=__PYTHON__ __APP_DIR__/sort_mail_daemon.py --account __ACCOUNT__ --folder INBOX --state-file __STATE_DIR__/sort_mail_daemon_state.json --log-file __LOG_DIR__/sort_mail_daemon.log
 Restart=always
 RestartSec=30
 NoNewPrivileges=true
 PrivateTmp=true
 ProtectSystem=full
 ProtectHome=true
-ReadWritePaths=__APP_DIR__ /var/lib/mailcat /var/log/mailcat
+ReadWritePaths=__APP_DIR__ __STATE_DIR__ __LOG_DIR__
 
 [Install]
 WantedBy=multi-user.target