Add MainHUD gameplay UI, net handshake updates, and server maintenance mode.

Introduce UiManager/MainHUD prefab flow, strengthen GameNet/WsClient, and add AIServer maintenance endpoint support.

Co-authored-by: Cursor <cursoragent@cursor.com>

AIServer/src/maintenance.hpp

@@ -0,0 +1,14 @@
+#pragma once
+#include <string>
+
+namespace maintenance {
+
+struct State {
+  bool enabled = false;
+  std::string message = "服务器维护中，请稍后再试。";
+};
+
+/// 读取 JSON：{"enabled":true,"message":"..."}；文件不存在则 enabled=false。
+State load(const std::string& path);
+
+}  // namespace maintenance