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

TechHelper.Client/Pages/Common/ExamGlobalInfoDialog.razor

@@ -22,7 +22,7 @@
     private IMudDialogInstance MudDialog { get; set; }
 
     [Parameter]
-    public AssignmentDto Assignment { get; set; } = new AssignmentDto();
+    public ExamDto Assignment { get; set; } = new ExamDto();
 
     private void Cancel() => MudDialog.Cancel();