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

backend/src/Health.Infrastructure/AI/ai_clients.cs

@@ -114,7 +114,7 @@ public sealed class VisionClient(HttpClient http, IConfiguration config)
 
         var messages = new List<ChatMessage>
         {
-            new() { Role = "user", Content = JsonSerializer.Serialize(contentParts, _jsonOptions) }
+            new() { Role = "user", Content = contentParts }
         };
 
         var request = new ChatCompletionRequest

backend/src/Health.Infrastructure/AI/open_ai_compatible_client.cs

@@ -174,8 +174,10 @@ public sealed class ChatCompletionRequest
 public sealed class ChatMessage
 {
     public string Role { get; set; } = string.Empty;
-    public string Content { get; set; } = string.Empty;
+    public object? Content { get; set; } = string.Empty;
+    [System.Text.Json.Serialization.JsonIgnore(Condition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull)]
     public string? ToolCallId { get; set; }
+    [System.Text.Json.Serialization.JsonIgnore(Condition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull)]
     public List<ToolCall>? ToolCalls { get; set; }
 }