重构项目结构，移除Assignment相关功能，优化Submission模块

Entities/DTO/Class/ClassUserDto.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace Entities.DTO
+{
+    public class ClassUserDto
+    {
+        public Guid StudentId { get; set; }
+        public string? Email { get; set; }
+        public string? DisplayName { get; set; }
+        public string? PhoneNumber { get; set; }
+        public string? Address { get; set; }
+        public bool TwoFactorEnabled { get; set; }
+        public bool EmailConfirmed { get; set; }
+    }
+}