feat: Abwesenheits-Hinweise, Fehlquote bei Zeugnisnoten, Gruppen-Dokumentation

- Schnellbewerten-Dialog: abwesende Schüler werden gedimmt und mit ihrem
  Anwesenheitsstatus statt der Aspektbeschriftung angezeigt, damit keine
  Mitarbeitsnote für nicht anwesende Schüler vergeben wird.
- Zeugnisnoten-Dialog: zeigt je Schüler die Fehlquote im gewählten
  Zeitraum, ab 50 % hervorgehoben (informativ, keine automatische
  Notenänderung).
- Gruppen-Tab "Dokumentation" (bisher Platzhalter) implementiert: listet
  alle Dokumentationseinträge der Gruppen-Schüler, mit Schüler-Filter und
  optionalem "Nur dieser Unterricht"-Schalter. Einträge aus anderen
  Lerngruppen werden standardmäßig mitangezeigt, aber gedimmt. Der
  Dokumentationsdialog bekommt dafür einen optionalen Schüler-Picker.

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

LehrerApp.Desktop.Tests/GroupDocumentationTabViewModelTests.cs

@@ -0,0 +1,147 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.ViewModels.Groups;
+using LehrerApp.Desktop.ViewModels.Students;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+/// Tests für den bisher als Platzhalter existierenden Gruppen-Tab "Dokumentation" (Nutzer-Feedback):
+/// listet Dokumentationseinträge aller Gruppen-Schüler, zeigt Einträge aus anderen Lerngruppen
+/// standardmäßig mit an (optisch abgesetzt statt versteckt), mit optionalem Filter darauf.
+public sealed class GroupDocumentationTabViewModelTests
+{
+    private static GroupDocumentationTabViewModel BuildVm(
+        List<Student> students, List<LearningGroup> groups, FakeDocumentation? docs = null)
+    {
+        var vm = new GroupDocumentationTabViewModel(docs ?? new FakeDocumentation(),
+            new FakeStudents(students), new FakeGroups(groups));
+        return vm;
+    }
+
+    [Fact]
+    public void Initialize_ZeigtEintraegeAllerGruppenschuelerSortiertNachDatumAbsteigend()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var ben = new Student { FirstName = "Ben", LastName = "Muster" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = group.Id, Title = "Alt", Date = new DateOnly(2025, 9, 1) });
+        docs.Add(new Documentation { StudentId = ben.Id, GroupId = group.Id, Title = "Neu", Date = new DateOnly(2025, 9, 10) });
+
+        var vm = BuildVm([anna, ben], [group], docs);
+        vm.Initialize(group.Id);
+
+        Assert.Equal(2, vm.Entries.Count);
+        Assert.Equal("Neu", vm.Entries[0].Model.Title);
+        Assert.Equal("Alt", vm.Entries[1].Model.Title);
+    }
+
+    [Fact]
+    public void Initialize_EintragAusAndererGruppe_WirdMitangezeigtAberAlsFremdMarkiert()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var otherGroup = new LearningGroup { Name = "Mathematik 9b" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = otherGroup.Id, Title = "Aus anderem Kurs", Date = new DateOnly(2025, 9, 1) });
+
+        var vm = BuildVm([anna], [group, otherGroup], docs);
+        vm.Initialize(group.Id);
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.False(entry.IsOwnGroup);
+        Assert.Equal("Mathematik 9b", entry.OtherGroupLabel);
+        Assert.True(entry.ContentOpacity < 1.0);
+    }
+
+    [Fact]
+    public void Initialize_EintragOhneGruppe_GiltAlsEigen()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = null, Title = "Allgemein", Date = new DateOnly(2025, 9, 1) });
+
+        var vm = BuildVm([anna], [group], docs);
+        vm.Initialize(group.Id);
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.True(entry.IsOwnGroup);
+    }
+
+    [Fact]
+    public void OnlyThisGroup_Aktiviert_BlendetEintraegeAusAnderenGruppenAus()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var otherGroup = new LearningGroup { Name = "Mathematik 9b" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = group.Id, Title = "Eigen", Date = new DateOnly(2025, 9, 1) });
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = otherGroup.Id, Title = "Fremd", Date = new DateOnly(2025, 9, 2) });
+
+        var vm = BuildVm([anna], [group, otherGroup], docs);
+        vm.Initialize(group.Id);
+        vm.OnlyThisGroup = true;
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.Equal("Eigen", entry.Model.Title);
+    }
+
+    [Fact]
+    public void SelectedStudentFilter_AufEinzelnenSchueler_FiltertListe()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var ben = new Student { FirstName = "Ben", LastName = "Muster" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = group.Id, Title = "Anna-Eintrag", Date = new DateOnly(2025, 9, 1) });
+        docs.Add(new Documentation { StudentId = ben.Id, GroupId = group.Id, Title = "Ben-Eintrag", Date = new DateOnly(2025, 9, 1) });
+
+        var vm = BuildVm([anna, ben], [group], docs);
+        vm.Initialize(group.Id);
+        vm.SelectedStudentFilter = vm.StudentFilterOptions.Single(s => s.Id == anna.Id);
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.Equal("Anna-Eintrag", entry.Model.Title);
+    }
+
+    [Fact]
+    public async Task AddDocumentation_SpeichertErgebnisUndLaedtNeu()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        var vm = BuildVm([anna], [group], docs);
+        vm.Initialize(group.Id);
+
+        vm.OnEditDocumentation = (groupId, options, editing) =>
+        {
+            Assert.Equal(group.Id, groupId);
+            Assert.Contains(options, o => o.Id == anna.Id);
+            Assert.Null(editing);
+            return Task.FromResult<Documentation?>(new Documentation
+            { StudentId = anna.Id, GroupId = groupId, Title = "Neu", Date = new DateOnly(2025, 9, 1) });
+        };
+
+        await vm.AddDocumentationCommand.ExecuteAsync(null);
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.Equal("Neu", entry.Model.Title);
+    }
+
+    [Fact]
+    public async Task DeleteDocumentation_NachBestaetigung_EntferntEintragAusListe()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = group.Id, Title = "Weg", Date = new DateOnly(2025, 9, 1) });
+        var vm = BuildVm([anna], [group], docs);
+        vm.Initialize(group.Id);
+        vm.OnConfirmDeleteDocumentation = _ => Task.FromResult(true);
+
+        await vm.DeleteDocumentationCommand.ExecuteAsync(vm.Entries[0]);
+
+        Assert.Empty(vm.Entries);
+    }
+}