Improve desktop UX and accessibility

LehrerApp.Desktop/ViewModels/Exams/ExamsOverviewViewModel.cs

@@ -43,6 +43,8 @@ public partial class ExamsOverviewViewModel : ObservableObject
     public Func<Exam, Task>? OnGradeExam { get; set; }
     public Func<Exam, Task>? OnEvaluateExam { get; set; }
     public Action<Guid>? OnNavigateToGroup { get; set; }
+    public Action? OnNavigateToGroups { get; set; }
+    public bool HasNoExams => Rows.Count == 0;
 
     public ExamsOverviewViewModel(IExamRepository exams, IExamResultRepository examResults,
         IGroupRepository groups, IGroupMembershipRepository memberships, GradingService grading,
@@ -85,6 +87,7 @@ public partial class ExamsOverviewViewModel : ObservableObject
             Rows.Add(row);
 
         EmptyHint = Rows.Count == 0 ? "Keine Klausuren angelegt." : "";
+        OnPropertyChanged(nameof(HasNoExams));
         SelectedRow = selectedId is { } id
             ? Rows.FirstOrDefault(r => r.Exam.Id == id) ?? Rows.FirstOrDefault()
             : Rows.FirstOrDefault();
@@ -173,6 +176,9 @@ public partial class ExamsOverviewViewModel : ObservableObject
         OnNavigateToGroup?.Invoke(SelectedRow.GroupId);
     }
 
+    [RelayCommand]
+    private void GoToGroups() => OnNavigateToGroups?.Invoke();
+
     [RelayCommand]
     private void ToggleApproval() => ToggleDate(SelectedRow?.Exam, e => e.ApprovalGrantedAt,
         (e, v) => e.ApprovalGrantedAt = v);