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.Infrastructure/Services/jwt_provider.cs

@@ -3,25 +3,17 @@ using Microsoft.IdentityModel.Tokens;
 using System.IdentityModel.Tokens.Jwt;
 using System.Security.Claims;
 using System.Security.Cryptography;
-using System.Text;
 
 namespace Health.Infrastructure.Services;
 
 /// <summary>
 /// JWT Token 生成与验证服务
 /// </summary>
-public sealed class JwtProvider
+public sealed class JwtProvider(IConfiguration configuration)
 {
-    private readonly string _secret;
-    private readonly string _issuer;
-    private readonly string _audience;
-
-    public JwtProvider(IConfiguration configuration)
-    {
-        _secret = configuration["JWT_SECRET"] ?? "dev-secret-key-change-in-production-min-32-chars!!";
-        _issuer = configuration["JWT_ISSUER"] ?? "health-manager";
-        _audience = configuration["JWT_AUDIENCE"] ?? "health-manager-app";
-    }
+    private readonly string _secret = configuration["JWT_SECRET"] ?? "dev-secret-key-change-in-production-min-32-chars!!";
+    private readonly string _issuer = configuration["JWT_ISSUER"] ?? "health-manager";
+    private readonly string _audience = configuration["JWT_AUDIENCE"] ?? "health-manager-app";
 
     /// <summary>
     /// 生成 access_token（30 分钟有效）