fix(mqtt): MQTT 连接成功后自动周期上报心跳（绑定页/主屏通用）

- mqtt.js：connect 后立即+每10s 上报 opc/display/heartbeat（client_id/tenant_id/page），
  绑定页(BindScreen)不再只连不报——admin 端屏幕管理即可看到在线大屏；
  断线 close 时清定时器，重连后自动恢复
- useMqttControl.js：移除重复心跳（避免双发），保留注册/状态上报/订阅逻辑

src/utils/useMqttControl.js

@@ -1,7 +1,7 @@
 import { useEffect } from 'react';
 import { useNavigate, useLocation } from 'react-router-dom';
 import { onMqttMessage, connectMqtt, publishMqtt, getClientId, useMqttStatus } from './mqtt';
-import { MQTT_TOPIC_HEARTBEAT, MQTT_TOPIC_COMMAND, MQTT_TOPIC_BIND, getApiBase as API_BASE } from '../config';
+import { MQTT_TOPIC_COMMAND, MQTT_TOPIC_BIND, getApiBase as API_BASE } from '../config';
 import { getTenantId, clearBinding } from './tenant';
 import { Sfx } from './sounds';
 import { PAGE_ORDER } from './pageNav';
@@ -75,23 +75,7 @@ export function useMqttControl() {
     return off;
   }, []);
 
-  // 大屏在线心跳（后端据此统计在线大屏数）
-  useEffect(() => {
-    connectMqtt();
-    const sendHeartbeat = () => {
-      if (mqttStatus === 'connected') {
-        publishMqtt(MQTT_TOPIC_HEARTBEAT, {
-          client_id: getClientId(),
-          tenant_id: getTenantId(),
-          page: location.pathname,
-          ts: Date.now(),
-        });
-      }
-    };
-    sendHeartbeat();
-    const hb = setInterval(sendHeartbeat, 10000);
-    return () => clearInterval(hb);
-  }, [mqttStatus, location.pathname]);
+  // 大屏在线心跳已由 mqtt.js 连接成功后自动周期上报（绑定页/主屏通用），此处不再重复发送
 
   // 页面状态即时上报：路由变化即通知服务端当前所在页面
   useEffect(() => {