feat(video_call): 添加视频通话远端视频显示功能

- 集成 agora_rtc_engine 依赖库
- 添加 RTCManager 实例用于管理实时通信
- 实现 _buildBackground 方法用于构建背景显示
- 在视频通话接通时显示远端用户视频流
- 当 remoteUid 为空时从 RTCManager 获取远端用户列表
- 保持原有头像显示逻辑作为默认背景
- 使用 Obx 监听响应式变量确保实时更新

lib/widget/message/video_call_overlay_widget.dart

@@ -1,8 +1,10 @@
+import 'package:agora_rtc_engine/agora_rtc_engine.dart';
 import 'package:cached_network_image/cached_network_image.dart';
 import 'package:dating_touchme_app/controller/message/call_controller.dart';
 import 'package:dating_touchme_app/extension/ex_widget.dart';
 import 'package:dating_touchme_app/generated/assets.dart';
 import 'package:dating_touchme_app/pages/message/video_call_page.dart';
+import 'package:dating_touchme_app/rtc/rtc_manager.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_screenutil/flutter_screenutil.dart';
 import 'package:get/get.dart';
@@ -30,6 +32,7 @@ class _VideoCallOverlayWidgetState extends State<VideoCallOverlayWidget> {
   Offset _position = Offset.zero;
   bool _isDragging = false;
   final CallController _callController = CallController.instance;
+  final RTCManager _rtcManager = RTCManager.instance;
 
   @override
   void initState() {
@@ -59,6 +62,76 @@ class _VideoCallOverlayWidgetState extends State<VideoCallOverlayWidget> {
     });
   }
 
+  /// 构建背景（视频或头像）
+  Widget _buildBackground() {
+    return Obx(() {
+      // 在 Obx 中访问响应式变量，确保建立监听关系
+      final callSession = _callController.currentCall.value;
+      final isVideoCall = callSession != null && callSession.callType == CallType.video;
+      final isConnected = callSession != null && _callController.callDurationSeconds.value > 0;
+      final remoteUid = _callController.remoteUid.value;
+      final remoteUsers = _rtcManager.remoteUsersNotifier.value;
+      
+      // 如果是视频通话且已接通，显示远端视频
+      if (isVideoCall && isConnected && remoteUid != null) {
+        final engine = _rtcManager.engine;
+        if (engine != null) {
+          final remoteVideoViewController = VideoViewController(
+            rtcEngine: engine,
+            canvas: VideoCanvas(uid: remoteUid),
+          );
+          return SizedBox(
+            width: 100.w,
+            height: 100.w,
+            child: AgoraVideoView(
+              controller: remoteVideoViewController,
+            ),
+          );
+        }
+      }
+      
+      // 如果 remoteUid 为空，尝试从 RTCManager 的远端用户列表中获取
+      if (isVideoCall && isConnected && remoteUid == null && remoteUsers.isNotEmpty) {
+        final firstRemoteUid = remoteUsers.first;
+        final engine = _rtcManager.engine;
+        if (engine != null) {
+          _callController.remoteUid.value = firstRemoteUid;
+          final remoteVideoViewController = VideoViewController(
+            rtcEngine: engine,
+            canvas: VideoCanvas(uid: firstRemoteUid),
+          );
+          return SizedBox(
+            width: 100.w,
+            height: 100.w,
+            child: AgoraVideoView(
+              controller: remoteVideoViewController,
+            ),
+          );
+        }
+      }
+      
+      // 默认显示头像
+      return Container(
+        width: 100.w,
+        height: 100.w,
+        color: Colors.black,
+        child: widget.targetAvatarUrl != null && widget.targetAvatarUrl!.isNotEmpty
+            ? CachedNetworkImage(
+                imageUrl: widget.targetAvatarUrl!,
+                fit: BoxFit.cover,
+                errorWidget: (context, url, error) => Image.asset(
+                  Assets.imagesUserAvatar,
+                  fit: BoxFit.cover,
+                ),
+              )
+            : Image.asset(
+                Assets.imagesUserAvatar,
+                fit: BoxFit.cover,
+              ),
+      );
+    });
+  }
+
   /// 格式化通话时长
   String _formatDuration(int seconds) {
     String twoDigits(int n) => n.toString().padLeft(2, '0');
@@ -123,25 +196,8 @@ class _VideoCallOverlayWidgetState extends State<VideoCallOverlayWidget> {
             borderRadius: BorderRadius.circular(8.w),
             child: Stack(
               children: [
-                // 背景：头像
-                Container(
-                  width: 100.w,
-                  height: 100.w,
-                  color: Colors.black,
-                  child: widget.targetAvatarUrl != null && widget.targetAvatarUrl!.isNotEmpty
-                      ? CachedNetworkImage(
-                          imageUrl: widget.targetAvatarUrl!,
-                          fit: BoxFit.cover,
-                          errorWidget: (context, url, error) => Image.asset(
-                            Assets.imagesUserAvatar,
-                            fit: BoxFit.cover,
-                          ),
-                        )
-                      : Image.asset(
-                          Assets.imagesUserAvatar,
-                          fit: BoxFit.cover,
-                        ),
-                ),
+                // 背景：视频通话接通时显示远端视频，否则显示头像
+                _buildBackground(),
                 // 半透明遮罩
                 Container(
                   color: Colors.black.withOpacity(0.4),