feat: 增加信令自动重连保活与控制端视频录制

- 信令客户端增加心跳保活与指数退避自动重连机制
- 服务端优化传输异常日志级别，客户端断开不再报错
- 被控端信令断开时保持服务运行并自动重连
- Flutter 控制端增加远程视频录制功能，兼容标准 WebRTC 模式切换
- 各端连接按钮增加“连接中”状态防重复点击

WebRTCControllerWeb/src/store/controllerStore.js

@@ -26,6 +26,7 @@ export const store = reactive({
 
   protoReady: false,
   signalingConnected: false,
+  signalingConnecting: false,
   registered: false,
   rtcConnected: false,
   dataChannelOpen: false,
@@ -60,20 +61,26 @@ export function connectSignaling() {
   const deviceId = store.deviceId.trim();
   if (!deviceId) { store.error = '请填写本机设备 ID'; return; }
 
+  store.signalingConnecting = true;
   signaling = new SignalingClient({
     serverUrl: store.serverUrl.trim(),
     deviceId,
     onConnected: () => {
       store.signalingConnected = true;
+      store.signalingConnecting = false;
       store.statusText = '已连接信令服务器';
       refreshDevices();
     },
     onDisconnected: () => {
       store.signalingConnected = false;
+      store.signalingConnecting = false;
       store.registered = false;
       store.statusText = '已断开信令连接';
     },
-    onError: (e) => { store.error = '信令错误: ' + e; },
+    onError: (e) => {
+      store.signalingConnecting = false;
+      store.error = '信令错误: ' + e;
+    },
     onMessage: handleSignalMessage,
   });
   signaling.connect();
@@ -172,6 +179,7 @@ export function disconnectSignaling() {
   disconnectDevice();
   if (signaling) { signaling.disconnect(); signaling = null; }
   store.signalingConnected = false;
+  store.signalingConnecting = false;
   store.registered = false;
   store.controlledDevices = [];
 }