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

Entities/DTO/Textbook/TextbookResponseDto.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+
+namespace Entities.DTO
+{
+    /// <summary>
+    /// 教材响应数据传输对象
+    /// </summary>
+    public class TextbookResponseDto
+    {
+        public Guid Id { get; set; }
+        public string Title { get; set; }
+        public string Grade { get; set; }
+        public string Publisher { get; set; }
+        public string SubjectArea { get; set; }
+        public string Description { get; set; }
+
+        // 导航属性
+        public int LessonCount { get; set; }
+        public List<string> LessonTitles { get; set; } = new List<string>();
+    }
+}