fix: 任务卡片移入对话流 + 综合信息卡片分组修复

- 新增 taskCard 消息类型，作为对话第一条消息
- 今日任务卡片从独立区域移至聊天流内
- AgentWelcomeCard 从 metadata 读 agent 不再全局共享
- 切换胶囊不会影响已发过的卡片

health_app/lib/providers/chat_provider.dart

@@ -5,7 +5,7 @@ import 'auth_provider.dart';
 import 'data_providers.dart';
 import '../utils/sse_handler.dart';
 
-enum MessageType { text, dataConfirm, medicationConfirm, dietAnalysis, reportAnalysis, quickOptions, agentWelcome }
+enum MessageType { text, dataConfirm, medicationConfirm, dietAnalysis, reportAnalysis, quickOptions, agentWelcome, taskCard }
 
 class ChatMessage {
   final String id;
@@ -115,7 +115,22 @@ class ChatNotifier extends Notifier<ChatState> {
   StreamSubscription<Map<String, dynamic>>? _subscription;
 
   @override
-  ChatState build() => const ChatState();
+  ChatState build() {
+    // 首次加载时插入今日任务卡片作为第一条消息
+    Future.microtask(() => insertTaskCard());
+    return const ChatState();
+  }
+
+  void insertTaskCard() {
+    if (state.messages.any((m) => m.type == MessageType.taskCard)) return;
+    state = state.copyWith(messages: [ChatMessage(
+      id: 'task_card',
+      role: 'assistant',
+      content: '',
+      createdAt: DateTime.now(),
+      type: MessageType.taskCard,
+    ), ...state.messages]);
+  }
 
   void setAgent(ActiveAgent a) {
     _subscription?.cancel();