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

api/routes/status.py

@@ -6,7 +6,7 @@ import time
 
 from fastapi import APIRouter
 
-from config_io import LOG_DB, load_config, save_config
+from config_io import LOG_DB, load_config, touch_restart_signal
 
 router = APIRouter()
 
@@ -53,7 +53,10 @@ def status() -> dict:
 
 @router.post("/proxy/restart")
 def restart_proxy() -> dict:
-    """config 파일 mtime 을 갱신해서 프록시 watcher 가 재로드하게 한다."""
-    cfg = load_config()
-    save_config(cfg)
-    return {"ok": True, "note": "config touched; proxy will reload"}
+    """프록시 listener 를 강제로 stop → start 시킨다.
+
+    config 가 바뀌지 않았어도 listener 자체를 재시작하기 때문에 단순
+    config reload 와는 다르다. (proxy watcher 가 별도 신호 파일을 폴링)
+    """
+    ts = touch_restart_signal()
+    return {"ok": True, "restart_signal_ts": ts}