feat: 添加学生提交系统功能

- 添加学生提交管理服务 (StudentSubmissionService, StudentSubmissionDetailService)
- 新增学生提交相关控制器 (StudentSubmissionController, StudentSubmissionDetailController)
- 添加学生提交数据传输对象 (StudentSubmissionDetailDto, StudentSubmissionSummaryDto)
- 新增学生提交相关页面组件 (StudentExamView, ExamDetailView, StudentCard等)
- 添加学生提交信息卡片组件 (SubmissionInfoCard, TeacherSubmissionInfoCard)
- 更新数据库迁移文件以支持提交系统

TechHelper.Client/Pages/Exam/ExamManager.razor

@@ -21,8 +21,6 @@ else
 <MudPaper Class="d-flex flex-wrap flex-grow-0 gap-4" Height="100%" Width="100%">
 	@foreach (var item in examDtos)
 	{
-		@* <ExamPreview AssignmentDto="item" Width="256px" Height="256px"> </ExamPreview> *@
-		<AssignmentInfoCard></AssignmentInfoCard>
 	}
 </MudPaper>