Fix: prevent school subject deletion if they have homework

WorkManagementTool/Components/SchoolSubject/SchoolSubjectDeletion.razor

@@ -5,7 +5,10 @@
 @using WorkManagementTool.Services
 @inject SchoolSubjectService SchoolSubjectService
 
-<button type="button" class="btn btn-danger" @onclick="Delete">-</button>
+@if (!hasHomeworks)
+{
+    <button type="button" class="btn btn-danger" @onclick="Delete">-</button>
+}
 
 @code {
     [Parameter]
@@ -14,6 +17,16 @@
     [Parameter]
     public EventCallback<int> OnDeleted { get; set; }
 
+    private bool hasHomeworks;
+
+    protected override async Task OnInitializedAsync()
+    {
+        var schoolSubject = await SchoolSubjectService.GetSchoolSubjectByIdAsync(Id);
+
+        hasHomeworks = schoolSubject is not null
+            && schoolSubject.Homeworks?.Count > 0;
+    }
+
     private async Task Delete(MouseEventArgs e)
     {
         await SchoolSubjectService.DeleteSchoolSubjectAsync(Id);