fix: 管理后台连接状态 + 修复按钮无响应

连接状态（正式）：
- /api/display/state 扩展：screens_online（大屏心跳统计，opc/display/heartbeat）+ last_command（最近指令与发布结果）
- 前端大屏每 10s 上报心跳（稳定 client_id），后端 30s TTL 统计在线大屏数
- 管理台新增「连接状态」卡片：后端/MQTT/在线大屏/最近指令，3s 轮询刷新

按钮无响应根治：
- admin.js 改为 document 事件委托（data-nav/data-control/data-action/复制/预览），不再依赖逐个绑定
- /static 与 /admin 响应加 no-cache 头，杜绝旧 JS/CSS 缓存导致的按钮失效
- SSE 不再自动重载页面（避免与操作回调双重重载清掉反馈）；立即播放改为 toast 不重载

src/config.js

@@ -17,3 +17,4 @@ export const MQTT_PASSWORD = window.__DPM_MQTT_PASS__ || env.VITE_MQTT_PASSWORD
 
 export const MQTT_TOPIC_COMMAND = 'opc/display/command';
 export const MQTT_TOPIC_TICK = 'opc/dashboard/tick';
+export const MQTT_TOPIC_HEARTBEAT = 'opc/display/heartbeat';

src/utils/mqtt.js

@@ -10,6 +10,7 @@ import { MQTT_URL, MQTT_TOPIC_COMMAND, MQTT_TOPIC_TICK, MQTT_USERNAME, MQTT_PASS
 
 let client = null;
 let status = 'connecting'; // connecting | connected | offline
+let clientId = null;
 const listeners = new Set();
 
 function notifyStatus() {
@@ -19,10 +20,11 @@ function notifyStatus() {
 export function connectMqtt() {
   if (client) return;
   try {
+    clientId = `dpm-screen-${Math.random().toString(16).slice(2, 8)}`;
     client = mqtt.connect(MQTT_URL, {
       reconnectPeriod: 3000,
       connectTimeout: 8000,
-      clientId: `dpm-screen-${Math.random().toString(16).slice(2, 8)}`,
+      clientId,
       clean: true,
       username: MQTT_USERNAME || undefined,
       password: MQTT_PASSWORD || undefined,
@@ -63,6 +65,22 @@ export function getMqttStatus() {
   return status;
 }
 
+/** 当前连接的稳定 client_id */
+export function getClientId() {
+  return clientId;
+}
+
+/** 发布消息（大屏心跳等） */
+export function publishMqtt(topic, payload) {
+  if (!client || status !== 'connected') return false;
+  try {
+    client.publish(topic, JSON.stringify(payload), { qos: 1 });
+    return true;
+  } catch (e) {
+    return false;
+  }
+}
+
 /** 订阅 MQTT 消息，返回取消订阅函数 */
 export function onMqttMessage(fn) {
   listeners.add(fn);

src/utils/useMqttControl.js

@@ -1,6 +1,7 @@
 import { useEffect } from 'react';
 import { useNavigate, useLocation } from 'react-router-dom';
-import { onMqttMessage, connectMqtt } from './mqtt';
+import { onMqttMessage, connectMqtt, publishMqtt, getClientId, useMqttStatus } from './mqtt';
+import { MQTT_TOPIC_HEARTBEAT } from '../config';
 import { PAGE_ORDER } from './pageNav';
 
 /* =========================================================
@@ -9,6 +10,7 @@ import { PAGE_ORDER } from './pageNav';
    · play / pause / next / prev / set_mode：媒体控制
    · alert：全局通知
    · show_card：展示信息卡片（AI 工具调用 / 管理端指令）
+   · 心跳：定时上报 opc/display/heartbeat（后端统计在线大屏）
    ========================================================= */
 
 const PAGE_ALIAS = {
@@ -28,7 +30,26 @@ function resolvePage(page) {
 export function useMqttControl() {
   const navigate = useNavigate();
   const location = useLocation();
+  const mqttStatus = useMqttStatus();
 
+  // 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(),
+          page: location.pathname,
+          ts: Date.now(),
+        });
+      }
+    };
+    sendHeartbeat();
+    const hb = setInterval(sendHeartbeat, 10000);
+    return () => clearInterval(hb);
+  }, [mqttStatus, location.pathname]);
+
+  // 控制指令订阅
   useEffect(() => {
     connectMqtt();
     const off = onMqttMessage((topic, cmd) => {