WIP (unstable): Klassenlehrer-Tab (fremde Klassenbucheinträge + Fehlzeiten der Klasse)

Neuer Tab in der Gruppendetailansicht (LearningGroup.IsClassTeacher-Flag), zeigt
Klassenbucheinträge anderer Lehrkräfte sowie Fehlzeiten der ganzen Klasse über alle
Fächer, pro Schüler*in und Tag zusammengefasst. Beides rein lesend, kein Übernehmen
in lokale Daten. Fehlzeiten-Bericht (AbsencePerStudent) adressiert die Klasse über
"KL"+getKlassen-ID, automatisch aufgelöst - noch nicht mit echtem WebUntis-Zugang
gegengeprüft (siehe TODO.md).

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

LehrerApp.Desktop/ViewModels/Groups/GroupViewModels.cs

@@ -251,6 +251,7 @@ public partial class GroupDetailViewModel : ObservableObject
     // bevor LoadGroup() läuft (siehe MainWindowViewModel.NavigateToGroupDetail), Group ist dann
     // kurzzeitig null — ein verschachtelter Pfad würde dafür jedes Mal einen Binding-Fehler loggen.
     public bool IsDifferentiated => Group?.IsDifferentiated ?? false;
+    public bool IsClassTeacher => Group?.IsClassTeacher ?? false;
     public bool IsReadOnly => Group is { IsActive: false };
     public bool IsEditable => Group is { IsActive: true };
     public string SubjectName { get; private set; } = "";
@@ -258,6 +259,7 @@ public partial class GroupDetailViewModel : ObservableObject
     partial void OnGroupChanged(LearningGroup? value)
     {
         OnPropertyChanged(nameof(IsDifferentiated));
+        OnPropertyChanged(nameof(IsClassTeacher));
         OnPropertyChanged(nameof(IsReadOnly));
         OnPropertyChanged(nameof(IsEditable));
         NotifyWriteCommands();
@@ -279,6 +281,7 @@ public partial class GroupDetailViewModel : ObservableObject
     public CompetencyOverviewTabViewModel CompetencyOverviewTab { get; }
     public SeatingPlanTabViewModel SeatingPlanTab { get; }
     public GroupDocumentationTabViewModel GroupDocumentationTab { get; }
+    public ClassTeacherTabViewModel ClassTeacherTab { get; }
     public Func<Task<bool>>? OnAddStudent { get; set; }
     public Func<StudentSummary, Task<bool>>? OnWithdrawStudent { get; set; }
     public Func<Guid, Task<bool>>? OnAddExam { get; set; }
@@ -295,7 +298,8 @@ public partial class GroupDetailViewModel : ObservableObject
         GroupOverviewViewModel overviewTab, ParticipationTabViewModel participationTab,
         GradeOverviewTabViewModel gradeOverviewTab,
         PlanningTabViewModel planningTab, CompetencyOverviewTabViewModel competencyOverviewTab,
-        SeatingPlanTabViewModel seatingPlanTab, GroupDocumentationTabViewModel groupDocumentationTab)
+        SeatingPlanTabViewModel seatingPlanTab, GroupDocumentationTabViewModel groupDocumentationTab,
+        ClassTeacherTabViewModel classTeacherTab)
     {
         _groups = groups; _students = students; _memberships = memberships; _subjects = subjects;
         _exams = exams; _grades = grades; _tasks = tasks;
@@ -306,6 +310,7 @@ public partial class GroupDetailViewModel : ObservableObject
         CompetencyOverviewTab = competencyOverviewTab;
         SeatingPlanTab = seatingPlanTab;
         GroupDocumentationTab = groupDocumentationTab;
+        ClassTeacherTab = classTeacherTab;
         SeatingPlanTab.OnAssessmentChanged = () =>
         {
             ParticipationTab.LoadSessions();
@@ -335,6 +340,7 @@ public partial class GroupDetailViewModel : ObservableObject
         CompetencyOverviewTab.Initialize(Group);
         SeatingPlanTab.Initialize(Group.Id, IsReadOnly);
         GroupDocumentationTab.Initialize(Group.Id);
+        ClassTeacherTab.Initialize(Group.Id);
     }
 
     private void ReloadExams()
@@ -877,6 +883,7 @@ public partial class AddGroupDialogViewModel : ObservableObject
     [ObservableProperty] private int? _hoursPerWeek;
     [ObservableProperty] private bool _isOwnClass;
     [ObservableProperty] private bool _isDifferentiated;
+    [ObservableProperty] private bool _isClassTeacher;
     [ObservableProperty] private bool _requiresLessonPlanning = true;
     [ObservableProperty] private int? _webUntisLessonId;
     [ObservableProperty] private string _nameError = "";
@@ -917,6 +924,7 @@ public partial class AddGroupDialogViewModel : ObservableObject
         HoursPerWeek = group.HoursPerWeek;
         IsOwnClass = group.IsOwnClass;
         IsDifferentiated = group.IsDifferentiated;
+        IsClassTeacher = group.IsClassTeacher;
         RequiresLessonPlanning = group.RequiresLessonPlanning;
         WebUntisLessonId = group.WebUntisLessonId;
         OnPropertyChanged(nameof(DialogTitle));
@@ -964,6 +972,7 @@ public partial class AddGroupDialogViewModel : ObservableObject
         Result.HoursPerWeek  = HoursPerWeek;
         Result.IsOwnClass    = IsOwnClass;
         Result.IsDifferentiated = IsDifferentiated;
+        Result.IsClassTeacher = IsClassTeacher;
         Result.RequiresLessonPlanning = RequiresLessonPlanning;
         Result.WebUntisLessonId = WebUntisLessonId;
         _groups.Save(Result);