Klausurtermine im Kalender (4.4.3) + Stundenplan: Ausnahme eintragen, Zahnrad-Settings-Link

- 4.4.3: Box "ANSTEHENDE KLAUSUREN" im Heute-Tab, gruppenübergreifend, nächste 21 Tage.
  IExamRepository um GetAll() erweitert.
- Nutzer-Feedback: "Vertretung eintragen" deckt auch Ausfall/Sondereinsatz ab, daher
  umbenannt in "Ausnahme eintragen". "Stundenplan bearbeiten" (redundant zum Bearbeiten-Tab)
  ersetzt durch Zahnrad-Button, der direkt in die Einstellungen springt.

LehrerApp.Desktop.Tests/TimetableViewModelTests.cs

@@ -148,13 +148,15 @@ public sealed class TimetableViewModelTests
     }
 
     [Fact]
-    public void ShowEditor_SchaltetAufBearbeiten_Tab()
+    public void OpenSettings_RuftOnNavigateToSettingsAuf()
     {
         var vm = BuildViewModel(new FakeTimetableSlots(), new FakeGroups([]));
+        var called = false;
+        vm.OnNavigateToSettings = () => called = true;
 
-        vm.ShowEditorCommand.Execute(null);
+        vm.OpenSettingsCommand.Execute(null);
 
-        Assert.Equal(1, vm.ActiveTabIndex);
+        Assert.True(called);
     }
 
     // ── "Heute"-Ansicht: Wochenraster (Nutzer-Feedback, zweite Iteration) ────────────────────
@@ -742,4 +744,68 @@ public sealed class TimetableViewModelTests
         var item = Assert.Single(vm.TodayItems);
         Assert.False(item.HasLessonTopic);
     }
+
+    // ── Anstehende Klausurtermine (4.4.3) ────────────────────────────────────
+
+    [Fact]
+    public void Load_ZeigtKlausurInnerhalbDesVorausblicks()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "Q1 Chemie" };
+        var exams = new FakeExams([new Exam { GroupId = group.Id, Date = today.AddDays(10), Title = "Klausur Nr. 2" }]);
+        var vm = BuildViewModel(new FakeTimetableSlots(), new FakeGroups([group]), exams: exams);
+
+        var item = Assert.Single(vm.UpcomingExams);
+        Assert.Equal("Q1 Chemie", item.GroupName);
+        Assert.Equal("Klausur Nr. 2", item.Title);
+        Assert.Equal(today.AddDays(10), item.Date);
+    }
+
+    [Fact]
+    public void Load_KlausurAusserhalbDesVorausblicks_WirdNichtGezeigt()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "Q1 Chemie" };
+        var exams = new FakeExams([new Exam { GroupId = group.Id, Date = today.AddDays(30), Title = "Klausur Nr. 3" }]);
+        var vm = BuildViewModel(new FakeTimetableSlots(), new FakeGroups([group]), exams: exams);
+
+        Assert.Empty(vm.UpcomingExams);
+    }
+
+    [Fact]
+    public void Load_VergangeneKlausur_WirdNichtGezeigt()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "Q1 Chemie" };
+        var exams = new FakeExams([new Exam { GroupId = group.Id, Date = today.AddDays(-1), Title = "Alte Klausur" }]);
+        var vm = BuildViewModel(new FakeTimetableSlots(), new FakeGroups([group]), exams: exams);
+
+        Assert.Empty(vm.UpcomingExams);
+    }
+
+    [Fact]
+    public void Load_MehrereKlausuren_SindNachDatumSortiert()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "Q1 Chemie" };
+        var exams = new FakeExams(
+        [
+            new Exam { GroupId = group.Id, Date = today.AddDays(15), Title = "Später" },
+            new Exam { GroupId = group.Id, Date = today.AddDays(3), Title = "Früher" },
+        ]);
+        var vm = BuildViewModel(new FakeTimetableSlots(), new FakeGroups([group]), exams: exams);
+
+        Assert.Equal(["Früher", "Später"], vm.UpcomingExams.Select(e => e.Title));
+    }
+
+    [Fact]
+    public void Load_HeutigeKlausur_IstImVorausblickEnthalten()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "Q1 Chemie" };
+        var exams = new FakeExams([new Exam { GroupId = group.Id, Date = today, Title = "Heute" }]);
+        var vm = BuildViewModel(new FakeTimetableSlots(), new FakeGroups([group]), exams: exams);
+
+        Assert.Single(vm.UpcomingExams);
+    }
 }