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

Entities/DTO/User/StudentExamDetailDto.cs

@@ -0,0 +1,21 @@
+using Entities.Contracts;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+    public record StudentExamDetailDto
+    {
+        public Guid Id { get; init; }
+        public string? DisplayName { get; init; }
+
+        public uint ErrorQuestionNum { get; init; }
+        public Dictionary<string, uint> ErrorQuestionTypes { get; init; } = new Dictionary<string, uint>();	
+        public Dictionary<SubjectAreaEnum, uint> SubjectAreaErrorQuestionDis { get; init; } = new Dictionary<SubjectAreaEnum, uint>();	
+        public Dictionary<byte, uint> LessonErrorDis { get; init; } = new Dictionary<byte, uint>();	
+        public float Score { get; init; }
+    }
+}