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

frontend/src/pages/Settings.jsx

@@ -31,16 +31,11 @@ export default function Settings() {
         <h2>프록시</h2>
         <label>
           리스닝 포트
-          <input
-            type="number"
-            value={cfg.proxy.listen_port}
-            onChange={(e) =>
-              setCfg({
-                ...cfg,
-                proxy: { ...cfg.proxy, listen_port: +e.target.value },
-              })
-            }
-          />
+          <input type="number" value={cfg.proxy.listen_port} disabled />
+          <span className="muted">
+            컨테이너 외부와 매핑된 포트라 docker-compose 의 ports 매핑과 함께 바꿔야 합니다.
+            대시보드에서는 표시만 합니다.
+          </span>
         </label>
         <label className="inline">
           <input
@@ -83,6 +78,21 @@ export default function Settings() {
       </section>
       <div className="actions">
         <button onClick={save}>저장</button>
+        <button
+          className="ghost"
+          onClick={async () => {
+            try {
+              await api.restart()
+              setMsg('프록시 listener 재시작 신호 전송')
+              setError(null)
+              setTimeout(() => setMsg(null), 2500)
+            } catch (e) {
+              setError(e.message)
+            }
+          }}
+        >
+          프록시 재시작
+        </button>
         {msg && <span className="ok">{msg}</span>}
       </div>
     </div>

frontend/src/styles.css

@@ -149,7 +149,14 @@ button {
 }
 button:hover { filter: brightness(1.1); }
 button.danger { background: var(--danger); }
+button.ghost {
+  background: transparent;
+  color: var(--text);
+  border: 1px solid var(--border);
+}
+button.ghost:hover { background: var(--panel-2); filter: none; }
 button.mt { margin-top: 12px; }
+input:disabled { opacity: 0.7; cursor: not-allowed; }
 
 input,
 select {