fix: real listener restart + remove listen_port editing from UI

Reviewer concerns:
1. POST /api/proxy/restart only saved config (reload), did not restart
   the listener. Now it touches data/restart.signal; proxy watcher
   polls that file separately and force-restarts the listener even
   when config is unchanged.
2. Editing proxy.listen_port via UI could break Docker port mapping
   (compose publishes 25565:25565 only). UI now shows it read-only;
   README documents how to change it together with compose.

- proxy/main.py: ProxyState.check_restart_signal() + watcher uses it
- api/config_io.py: touch_restart_signal() helper
- api/routes/status.py: /api/proxy/restart -> touch_restart_signal()
- frontend Settings: disabled listen_port input + 프록시 재시작 button
- README + .gitignore updated

.gitignore

@@ -3,6 +3,7 @@ data/config.json
 data/logs.db
 data/logs.db-wal
 data/logs.db-shm
+data/restart.signal
 
 # 노드/파이썬 빌드 산출물
 node_modules/