fix: VLM识别修复 + 饮食CRUD + 侧边栏美化 + UI优化

- VLM: ChatMessage.Content string→object, 修复视觉content双重序列化
- 饮食: diet/medication端点 record→手动JSON解析, 修复循环引用
- 饮食记录: 左滑删除 + 去评分 + AI饮食评语(DeepSeek)
- 侧边栏: 功能区统一Row+Expanded, 服务包compact模式
- 历史对话: 点击加载历史消息
- 登录页: 居中布局, 去底部空白
- UI: 主色加深#6C5CE7, maxWidth:1024防图片超大

health_app/lib/providers/chat_provider.dart

@@ -82,7 +82,7 @@ final conversationListProvider = FutureProvider<List<ConversationItem>>((ref) as
   if (token == null) return [];
   
   try {
-    final res = await api.get('/api/conversations');
+    final res = await api.get('/api/ai/conversations');
     final list = res.data['data'] as List? ?? [];
     return list.map((item) {
       final data = item as Map<String, dynamic>;
@@ -177,6 +177,36 @@ class ChatNotifier extends Notifier<ChatState> {
     state = state.copyWith(activeAgent: a);
   }
 
+  Future<void> loadConversation(String convId) async {
+    _subscription?.cancel();
+    try {
+      final api = ref.read(apiClientProvider);
+      final res = await api.get('/api/ai/conversations/$convId');
+      final rawMessages = (res.data['data'] as List?) ?? [];
+      final messages = rawMessages.map((m) {
+        final map = m as Map<String, dynamic>;
+        return ChatMessage(
+          id: map['id']?.toString() ?? '',
+          role: map['role']?.toString() ?? 'user',
+          content: map['content']?.toString() ?? '',
+          createdAt: DateTime.tryParse(map['createdAt']?.toString() ?? '') ?? DateTime.now(),
+          type: MessageType.text,
+        );
+      }).toList();
+
+      // 从对话列表中找到对应的 agent 类型
+      final convList = ref.read(conversationListProvider).value ?? [];
+      final conv = convList.firstWhere((c) => c.id == convId, orElse: () => ConversationItem(id: convId, title: '', lastMessage: '', updatedAt: DateTime.now(), agent: ActiveAgent.default_));
+
+      state = state.copyWith(
+        messages: messages,
+        conversationId: convId,
+        activeAgent: conv.agent,
+      );
+      ref.read(selectedAgentProvider.notifier).select(conv.agent);
+    } catch (_) {}
+  }
+
   void insertAgentWelcome(ActiveAgent agent) {
     state = state.copyWith(messages: [...state.messages, ChatMessage(
       id: 'welcome_${agent.name}_${DateTime.now().millisecondsSinceEpoch}',