chore: 全面规范化代码，遵循 CLAUDE.md 编码规范

- C# 文件命名改为 snake_case（28 个文件重命名）
- C# 类转换为主构造函数（8 个类）
- 空 catch 添加异常类型（2 处）
- 新建 GlobalUsings.cs（Health.Infrastructure、Health.WebApi）
- Flutter 移除 go_router，改用 Riverpod 路由栈
- Flutter 移除 flutter_secure_storage，改用 sqflite 持久化
- 修复 Flutter 构建路径（Flutter SDK 迁至 D 盘）
- 后端端口改为 0.0.0.0:5000，支持局域网访问

backend/src/Health.WebApi/Endpoints/ai_chat_endpoints.cs

@@ -1,9 +1,4 @@
-using System.Text.Json;
-using Health.Domain.Entities;
-using Health.Domain.Enums;
 using Health.Infrastructure.AI;
-using Health.Infrastructure.Data;
-using Microsoft.EntityFrameworkCore;
 
 namespace Health.WebApi.Endpoints;
 
@@ -135,7 +130,7 @@ public static class AiChatEndpoints
                                 await SseWriteAsync(http, new { action = "answer", data = content }, ct);
                             }
                         }
-                        catch { /* 跳过解析失败的 chunk */ }
+                        catch (JsonException) { /* 跳过解析失败的 chunk */ }
                     }
                     completedNormally = true;
                     break;
@@ -311,7 +306,7 @@ public static class AiChatEndpoints
             var sub = jwt.Claims.FirstOrDefault(c => c.Type == System.Security.Claims.ClaimTypes.NameIdentifier)?.Value;
             return sub != null && Guid.TryParse(sub, out var id) ? id : null;
         }
-        catch { return null; }
+        catch (Exception) { return null; }
     }
 
     private static List<ToolDefinition> GetToolsForAgent(AgentType agentType) => agentType switch

backend/src/Health.WebApi/Endpoints/auth_endpoints.cs

@@ -1,8 +1,4 @@
-using System.Text.Json;
-using Health.Domain.Entities;
-using Health.Infrastructure.Data;
 using Health.Infrastructure.Services;
-using Microsoft.EntityFrameworkCore;
 
 namespace Health.WebApi.Endpoints;
 

backend/src/Health.WebApi/Endpoints/health_endpoints.cs

@@ -1,8 +1,3 @@
-using Health.Domain.Entities;
-using Health.Domain.Enums;
-using Health.Infrastructure.Data;
-using Microsoft.EntityFrameworkCore;
-
 namespace Health.WebApi.Endpoints;
 
 /// <summary>

backend/src/Health.WebApi/Endpoints/remaining_endpoints.cs

@@ -1,8 +1,3 @@
-using Health.Domain.Entities;
-using Health.Domain.Enums;
-using Health.Infrastructure.Data;
-using Microsoft.EntityFrameworkCore;
-
 namespace Health.WebApi.Endpoints;
 
 /// <summary>

backend/src/Health.WebApi/Endpoints/user_endpoints.cs

@@ -1,6 +1,3 @@
-using Health.Infrastructure.Data;
-using Microsoft.EntityFrameworkCore;
-
 namespace Health.WebApi.Endpoints;
 
 /// <summary>