fix: 前端 WebSocket URL 改为环境变量或自动推导，消除 localhost 硬编码

frontend/src/lib/websocket.ts

@@ -6,7 +6,10 @@
 
 import type { ClientMessage, ServerMessage, WsMessage } from "../types";
 
-const WS_URL = "ws://localhost:8080/ws";
+// WebSocket 地址：优先使用环境变量，否则基于当前页面地址自动推导
+const WS_URL =
+  import.meta.env.VITE_WS_URL ||
+  `${window.location.protocol === "https:" ? "wss:" : "ws:"}//${window.location.host}/ws`;
 const PING_INTERVAL = 30_000;   // 30 秒心跳
 const MAX_RECONNECT_DELAY = 30_000; // 最大重连延迟 30 秒