Klausurverwaltung: Anlegen, Bearbeiten, Duplizieren, Status, Aufgaben

Implementiert TODO 1.1 (Klausur anlegen/verwalten) und die dazu passenden
Teile von 1.2 (Aufgabenstruktur), da beides im selben Dialog verwaltet wird:

- ExamDialog: Klausur anlegen/bearbeiten/duplizieren mit Aufgaben-Editor
  (Nr./Titel/Maximalpunkte/Gewichtung, sortierbar), Gesamtpunkte-Anzeige
  mit Warnung bei 0 Punkten, und Kompetenz-Zuordnung je Aufgabe.
- Klausuren-Tab: Kontextmenü und Toolbar für Bearbeiten/Duplizieren/
  Löschen (mit Rückfrage), farbcodierte Status-Spalte, SplitButton für
  Statuswechsel (Klick = nächster Status, Dropdown = manuelles Setzen).
- ExamRepository.Delete löscht zugehörige ExamResults kaskadierend.
- Gewichtung ist standardmäßig ausgeblendet (Checkbox zum Einblenden),
  NumericUpDown-Felder ohne Spinner-Buttons wegen eines Layout-Bugs im
  Avalonia-Standardtemplate, der das interne Textfeld auf wenige Pixel
  schrumpfen ließ.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

LehrerApp.Desktop/ViewModels/Groups/GroupViewModels.cs

@@ -165,12 +165,17 @@ public partial class GroupDetailViewModel : ObservableObject
     [ObservableProperty] private int _studentCount;
     [ObservableProperty] private int _activeTabIndex = 0;
     [ObservableProperty] private StudentSummary? _selectedStudent;
+    [ObservableProperty] private ExamSummary? _selectedExam;
 
     public ObservableCollection<StudentSummary> Students { get; } = [];
     public ObservableCollection<ExamSummary> Exams { get; } = [];
 
     public ParticipationTabViewModel ParticipationTab { get; }
     public Func<Task<bool>>? OnAddStudent { get; set; }
+    public Func<Guid, Task<bool>>? OnAddExam { get; set; }
+    public Func<Exam, Task<bool>>? OnEditExam { get; set; }
+    public Func<Exam, Task<bool>>? OnDuplicateExam { get; set; }
+    public Func<ExamSummary, Task<bool>>? OnConfirmDeleteExam { get; set; }
 
     public GroupDetailViewModel(IGroupRepository groups, IStudentRepository students,
         IEnrollmentRepository enrollments, IExamRepository exams, IGradeRepository grades,
@@ -190,9 +195,17 @@ public partial class GroupDetailViewModel : ObservableObject
                         $"{(Group.Type == GroupType.Class ? "Klasse" : "Kurs")} · " +
                         $"Stufe {Group.GradeLevel} · Noten {(Group.GradingSystem == GradingSystem.Grades1To6 ? "1–6" : "0–15")}";
         LoadStudents();
+        ReloadExams();
+        ParticipationTab.Initialize(Group.Id, Group.SchoolYear);
+    }
+
+    private void ReloadExams()
+    {
+        if (Group is null) return;
+        var selectedId = SelectedExam?.Id;
         Exams.Clear();
         foreach (var e in _exams.GetByGroup(Group.Id)) Exams.Add(new ExamSummary(e));
-        ParticipationTab.Initialize(Group.Id, Group.SchoolYear);
+        SelectedExam = Exams.FirstOrDefault(e => e.Id == selectedId);
     }
 
     public void LoadStudents()
@@ -240,7 +253,87 @@ public partial class GroupDetailViewModel : ObservableObject
 
     private bool HasSelectedStudent() => SelectedStudent is not null;
 
-    [RelayCommand] private void AddExam()  { /* TODO */ }
+    [RelayCommand]
+    private async Task AddExam()
+    {
+        if (Group is null || OnAddExam is null) return;
+        var saved = await OnAddExam(Group.Id);
+        if (saved) ReloadExams();
+    }
+
+    [RelayCommand(CanExecute = nameof(HasSelectedExam))]
+    private async Task EditExam()
+    {
+        if (SelectedExam is null || OnEditExam is null) return;
+        var exam = _exams.GetById(SelectedExam.Id);
+        if (exam is null) return;
+        var id = exam.Id;
+        var saved = await OnEditExam(exam);
+        if (saved)
+        {
+            ReloadExams();
+            SelectedExam = Exams.FirstOrDefault(e => e.Id == id);
+        }
+    }
+
+    [RelayCommand(CanExecute = nameof(HasSelectedExam))]
+    private async Task DuplicateExam()
+    {
+        if (SelectedExam is null || OnDuplicateExam is null) return;
+        var exam = _exams.GetById(SelectedExam.Id);
+        if (exam is null) return;
+        var saved = await OnDuplicateExam(exam);
+        if (saved) ReloadExams();
+    }
+
+    [RelayCommand(CanExecute = nameof(HasSelectedExam))]
+    private async Task DeleteExam()
+    {
+        if (SelectedExam is null || OnConfirmDeleteExam is null) return;
+        var selected = SelectedExam;
+        if (!await OnConfirmDeleteExam(selected)) return;
+        _exams.Delete(selected.Id);
+        SelectedExam = null;
+        ReloadExams();
+    }
+
+    [RelayCommand(CanExecute = nameof(HasSelectedExam))]
+    private void AdvanceExamStatus()
+    {
+        if (SelectedExam is null) return;
+        SetExamStatus(SelectedExam.Status switch
+        {
+            ExamStatus.Planned   => ExamStatus.Conducted,
+            ExamStatus.Conducted => ExamStatus.Graded,
+            ExamStatus.Graded    => ExamStatus.Returned,
+            _                    => SelectedExam.Status,
+        });
+    }
+
+    [RelayCommand(CanExecute = nameof(HasSelectedExam))]
+    private void SetExamStatus(ExamStatus status)
+    {
+        if (SelectedExam is null) return;
+        var exam = _exams.GetById(SelectedExam.Id);
+        if (exam is null) return;
+        exam.Status = status;
+        _exams.Save(exam);
+        var id = exam.Id;
+        ReloadExams();
+        SelectedExam = Exams.FirstOrDefault(e => e.Id == id);
+    }
+
+    partial void OnSelectedExamChanged(ExamSummary? value)
+    {
+        EditExamCommand.NotifyCanExecuteChanged();
+        DuplicateExamCommand.NotifyCanExecuteChanged();
+        DeleteExamCommand.NotifyCanExecuteChanged();
+        AdvanceExamStatusCommand.NotifyCanExecuteChanged();
+        SetExamStatusCommand.NotifyCanExecuteChanged();
+    }
+
+    private bool HasSelectedExam() => SelectedExam is not null;
+
     [RelayCommand] private void Refresh()  { if (Group is not null) LoadGroup(Group.Id); }
 }
 
@@ -277,10 +370,13 @@ public class ExamSummary
     public Guid Id { get; }
     public string Title { get; }
     public string Date { get; }
+    public ExamStatus Status { get; }
     public string StatusLabel { get; }
+    public string StatusColorHex { get; }
+
     public ExamSummary(Core.Models.Exam e)
     {
-        Id = e.Id; Title = e.Title; Date = e.Date.ToString("dd.MM.yyyy");
+        Id = e.Id; Title = e.Title; Date = e.Date.ToString("dd.MM.yyyy"); Status = e.Status;
         StatusLabel = e.Status switch
         {
             ExamStatus.Planned   => "Geplant",
@@ -289,6 +385,14 @@ public class ExamSummary
             ExamStatus.Returned  => "Zurückgegeben",
             _ => "",
         };
+        StatusColorHex = e.Status switch
+        {
+            ExamStatus.Planned   => "#9E9E9E",
+            ExamStatus.Conducted => "#FB8C00",
+            ExamStatus.Graded    => "#43A047",
+            ExamStatus.Returned  => "#1E88E5",
+            _ => "#9E9E9E",
+        };
     }
 }