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/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 {