fix: 停止机器人后UI消失 & UDP send error日志刷屏

- botDisconnected事件改为更新状态而非删除，停止机器人后正确显示离线状态
- 节流UDP send error日志，2秒内重复错误只输出一条汇总

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/web/websocket.ts

@@ -65,7 +65,11 @@ export function setupWebSocket(
     };
 
     const onDisconnected = () => {
-      broadcast({ type: "botDisconnected", botId: bot.id });
+      broadcast({
+        type: "botDisconnected",
+        botId: bot.id,
+        status: bot.getStatus(),
+      });
     };
 
     bot.on("stateChange", onStateChange);