feat: Komfort-Features für Unterrichtsplanung, Zeiterfassung und Dashboard

- Neue-Stunde-Dialog: Datum wird beim Anlegen anhand des Stundenplans
  und der letzten Stunde der Einheit vorbelegt statt auf "heute"
  (behebt eine falsch erkannte Doppelstunde, wenn "heute" nicht auf
  den passenden Wochentag fiel).
- Zeiterfassung: Button "Unterrichtszeit heute übernehmen" schlägt
  Start/Ende aus dem heutigen Stundenplan inkl. Puffer davor/danach vor.
- Dashboard: neue Kachel "Ungeplante Stunden" erinnert an Stunden ohne
  Thema für heute/morgen, mit Opt-out je Gruppe ("Benötigt
  Unterrichtsplanung"), Doppelstunden-Erkennung (keine doppelte Meldung
  für die zweite Periode) und Berücksichtigung von Stundenausfall.

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

LehrerApp.Desktop.Tests/WorkloadViewModelTests.cs

@@ -1,3 +1,4 @@
+using LehrerApp.Core.Interfaces;
 using LehrerApp.Core.Models;
 using LehrerApp.Core.Services;
 using LehrerApp.Desktop.ViewModels.Workload;
@@ -288,14 +289,16 @@ public sealed class AddEditWorkTaskDialogViewModelTests
 
 public sealed class TimeTrackingViewModelTests
 {
+    private static TimeTrackingViewModel BuildVm(ITimeEntryRepository entries, IWorkTaskRepository tasks,
+        FakeTimetableSlots? slots = null) =>
+        new(entries, tasks, slots ?? new FakeTimetableSlots(), TestSupport.BuildPeriodScheduleService());
+
     [Fact]
     public void StartStopTimer_SpeichertZeiteintragMitMindestensEinerMinute()
     {
         var entries = new FakeTimeEntries();
-        var vm = new TimeTrackingViewModel(entries, new FakeWorkTasks())
-        {
-            SelectedTimerCategory = TaskCategoryDisplay.Label(TaskCategory.Correction),
-        };
+        var vm = BuildVm(entries, new FakeWorkTasks());
+        vm.SelectedTimerCategory = TaskCategoryDisplay.Label(TaskCategory.Correction);
 
         vm.StartTimerCommand.Execute(null);
         Assert.True(vm.IsTimerRunning);
@@ -314,7 +317,7 @@ public sealed class TimeTrackingViewModelTests
         var task = new WorkTask { Title = "T", Category = TaskCategory.Meeting };
         var tasks = new FakeWorkTasks();
         tasks.Add(task);
-        var vm = new TimeTrackingViewModel(new FakeTimeEntries(), tasks);
+        var vm = BuildVm(new FakeTimeEntries(), tasks);
 
         vm.SelectedTimerTask = task;
 
@@ -328,7 +331,7 @@ public sealed class TimeTrackingViewModelTests
         entries.Add(new TimeEntry { Category = "X", Date = DateOnly.FromDateTime(DateTime.Today), DurationMinutes = 30 });
         entries.Add(new TimeEntry { Category = "X", Date = DateOnly.FromDateTime(DateTime.Today).AddDays(-30), DurationMinutes = 45 });
 
-        var vm = new TimeTrackingViewModel(entries, new FakeWorkTasks());
+        var vm = BuildVm(entries, new FakeWorkTasks());
 
         Assert.Single(vm.WeekEntries);
         Assert.Equal(30, vm.WeekEntries[0].Model.DurationMinutes);
@@ -343,7 +346,7 @@ public sealed class TimeTrackingViewModelTests
         entries.Add(new TimeEntry { Category = "Korrektur", Date = today, DurationMinutes = 20 });
         entries.Add(new TimeEntry { Category = "Verwaltung", Date = today, DurationMinutes = 10 });
 
-        var vm = new TimeTrackingViewModel(entries, new FakeWorkTasks());
+        var vm = BuildVm(entries, new FakeWorkTasks());
 
         var korrektur = vm.CategorySummaries.Single(s => s.Category == "Korrektur");
         Assert.Equal("50 min", korrektur.MinutesDisplay);
@@ -356,7 +359,7 @@ public sealed class TimeTrackingViewModelTests
     {
         var entries = new FakeTimeEntries();
         entries.Add(new TimeEntry { Category = "X", Date = DateOnly.FromDateTime(DateTime.Today), DurationMinutes = 30 });
-        var vm = new TimeTrackingViewModel(entries, new FakeWorkTasks());
+        var vm = BuildVm(entries, new FakeWorkTasks());
 
         vm.DeleteEntryCommand.Execute(vm.WeekEntries[0]);
 
@@ -367,7 +370,7 @@ public sealed class TimeTrackingViewModelTests
     public async Task AddEntry_RuftOnAddEntryAufUndSpeichertErgebnis()
     {
         var entries = new FakeTimeEntries();
-        var vm = new TimeTrackingViewModel(entries, new FakeWorkTasks());
+        var vm = BuildVm(entries, new FakeWorkTasks());
         vm.OnAddEntry = () => Task.FromResult<TimeEntry?>(
             new TimeEntry { Category = "X", Date = DateOnly.FromDateTime(DateTime.Today), DurationMinutes = 15 });
 
@@ -375,6 +378,64 @@ public sealed class TimeTrackingViewModelTests
 
         Assert.Single(vm.WeekEntries);
     }
+
+    // ── Unterrichtszeit-Vorschlag (Nutzer-Feedback: Zeiterfassung soll bei der Unterrichtszeit
+    // nachfragen statt sie komplett automatisch zu erfassen) ────────────────────────────────────
+
+    [Fact]
+    public void HasTeachingTimeSuggestionToday_OhneStundenplanHeute_IstFalse()
+    {
+        var vm = BuildVm(new FakeTimeEntries(), new FakeWorkTasks()); // leere FakeTimetableSlots
+
+        Assert.False(vm.HasTeachingTimeSuggestionToday);
+    }
+
+    [Fact]
+    public async Task SuggestTeachingTime_SchlaegtBeginnMitPufferVorErsterUndNachLetzterPeriodeVor()
+    {
+        var today = DateTime.Today.DayOfWeek;
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { Weekday = today, PeriodNumber = 1 });
+        slots.Add(new TimetableSlot { Weekday = today, PeriodNumber = 2 });
+        var periodSchedule = TestSupport.BuildPeriodScheduleService();
+        periodSchedule.SetPeriods([
+            new PeriodTimeEntry { PeriodNumber = 1, Start = new TimeOnly(7, 50), End = new TimeOnly(8, 35) },
+            new PeriodTimeEntry { PeriodNumber = 2, Start = new TimeOnly(8, 35), End = new TimeOnly(9, 20) },
+        ]);
+        var entries = new FakeTimeEntries();
+        var vm = new TimeTrackingViewModel(entries, new FakeWorkTasks(), slots, periodSchedule);
+
+        Assert.True(vm.HasTeachingTimeSuggestionToday);
+
+        (TimeOnly Start, TimeOnly End)? suggested = null;
+        vm.OnSuggestTeachingTime = (start, end) =>
+        {
+            suggested = (start, end);
+            return Task.FromResult<TimeEntry?>(new TimeEntry
+            {
+                Category = "Unterricht", Date = DateOnly.FromDateTime(DateTime.Today),
+                StartTime = start, EndTime = end, DurationMinutes = (int)(end - start).TotalMinutes,
+            });
+        };
+
+        await vm.SuggestTeachingTimeCommand.ExecuteAsync(null);
+
+        Assert.Equal(new TimeOnly(7, 35), suggested!.Value.Start); // 7:50 - 15 Min Puffer
+        Assert.Equal(new TimeOnly(9, 30), suggested.Value.End);    // 9:20 + 10 Min Puffer
+        Assert.Single(entries.GetByDate(DateOnly.FromDateTime(DateTime.Today)));
+    }
+
+    [Fact]
+    public async Task SuggestTeachingTime_OhneStundenplanHeute_TutNichts()
+    {
+        var vm = BuildVm(new FakeTimeEntries(), new FakeWorkTasks());
+        var called = false;
+        vm.OnSuggestTeachingTime = (_, _) => { called = true; return Task.FromResult<TimeEntry?>(null); };
+
+        await vm.SuggestTeachingTimeCommand.ExecuteAsync(null);
+
+        Assert.False(called);
+    }
 }
 
 public sealed class AddTimeEntryDialogViewModelTests