Sidebar-Kompaktmodus polieren, Dashboard-Kalender ergänzen

- MainWindow: App-Titel durch ein kompaktes Logo-Badge ersetzt; im
  minimierten Zustand der Sidebar werden Nav-Icons automatisch größer
  und zentriert, Beschriftungen sowie Abschnittsüberschriften blenden
  aus (Classes-Binding auf DrawerPage.IsOpen).
- LearningGroup.IsOwnClass: neues Feld, per Checkbox im Gruppen-Dialog
  editierbar ("Gehört zu meiner Klasse"), mehrere Gruppen markierbar.
- Dashboard: kleiner Monatskalender mit Monatsnavigation, dezenter
  Einfärbung für Unterrichtstage, Klausur-Marker und Rahmen-Hervor-
  hebung für Tage der eigenen Klasse, inkl. Tooltip und Legende.
  Feste Kartenposition/-breite, damit weder Monatsnamenlänge noch die
  wachsende Lerngruppen-Liste das Layout verschieben.
- TODO.md: geplante Kalender-Detailansicht (Tag auswählen → Termine
  des Tages mit Sprung in die jeweilige Ansicht) unter 9.8 vermerkt.

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

LehrerApp.Desktop/ViewModels/DashboardViewModel.cs

@@ -10,34 +10,44 @@ namespace LehrerApp.Desktop.ViewModels;
 
 public partial class DashboardViewModel : ObservableObject
 {
+    private static readonly CultureInfo De = new("de-DE");
+
     private readonly IGroupRepository _groups;
     private readonly ILessonRepository _lessons;
+    private readonly IExamRepository _exams;
     private readonly IWorkTaskRepository _tasks;
     private readonly SchoolYearService _sy;
 
     [ObservableProperty] private string _greeting = "";
     [ObservableProperty] private string _currentDate = "";
     [ObservableProperty] private string _currentSchoolYear = "";
+    [ObservableProperty] private DateOnly _calendarMonth = FirstOfMonth(DateTime.Today);
+
+    public string CalendarMonthLabel => CalendarMonth.ToString("MMMM yyyy", De);
 
     public ObservableCollection<LessonItem> TodaysLessons { get; } = [];
     public ObservableCollection<TaskItem> OpenTasks { get; } = [];
     public ObservableCollection<GroupChip> CurrentGroups { get; } = [];
+    public ObservableCollection<CalendarDayCell> CalendarDays { get; } = [];
+    public static string[] CalendarWeekdayHeaders { get; } = ["Mo", "Di", "Mi", "Do", "Fr", "Sa", "So"];
 
     // Navigation-Callback – wird von App.axaml.cs verdrahtet
     public Action<Guid>? OnNavigateToGroup { get; set; }
 
     public DashboardViewModel(IGroupRepository groups, ILessonRepository lessons,
-        IWorkTaskRepository tasks, SchoolYearService sy)
+        IExamRepository exams, IWorkTaskRepository tasks, SchoolYearService sy)
     {
-        _groups = groups; _lessons = lessons; _tasks = tasks; _sy = sy;
+        _groups = groups; _lessons = lessons; _exams = exams; _tasks = tasks; _sy = sy;
         Load();
     }
 
+    private static DateOnly FirstOfMonth(DateTime d) => new(d.Year, d.Month, 1);
+
     private void Load()
     {
         var now   = DateTime.Now;
         var today = DateOnly.FromDateTime(now);
-        CurrentDate        = now.ToString("dddd, d. MMMM yyyy", new CultureInfo("de-DE"));
+        CurrentDate        = now.ToString("dddd, d. MMMM yyyy", De);
         CurrentSchoolYear  = _sy.CurrentSchoolYear();
         Greeting = now.Hour < 12 ? "Guten Morgen" : now.Hour < 18 ? "Guten Tag" : "Guten Abend";
 
@@ -61,12 +71,120 @@ public partial class DashboardViewModel : ObservableObject
         CurrentGroups.Clear();
         foreach (var g in groups.Values.OrderBy(g => g.Name))
             CurrentGroups.Add(new() { GroupId = g.Id, Name = g.Name, Subject = g.Subject ?? "" });
+
+        CalendarMonth = FirstOfMonth(now);
+        LoadCalendar();
+    }
+
+    private void LoadCalendar()
+    {
+        CalendarDays.Clear();
+        var today = DateOnly.FromDateTime(DateTime.Today);
+
+        // Rasterstart: Montag der Woche, die den 1. des Monats enthält – 6 Wochen (42 Tage) Raster.
+        var firstOfMonth = CalendarMonth;
+        var mondayOffset = ((int)firstOfMonth.DayOfWeek + 6) % 7; // Montag=0 ... Sonntag=6
+        var gridStart    = firstOfMonth.AddDays(-mondayOffset);
+        var gridEnd      = gridStart.AddDays(41);
+
+        var groups = _groups.GetBySchoolYear(_sy.CurrentSchoolYear());
+        var byDay  = new Dictionary<DateOnly, DayAgg>();
+
+        DayAgg Agg(DateOnly d)
+        {
+            if (!byDay.TryGetValue(d, out var agg)) byDay[d] = agg = new DayAgg();
+            return agg;
+        }
+
+        foreach (var g in groups)
+        {
+            foreach (var lesson in _lessons.GetByGroupAndRange(g.Id, gridStart, gridEnd))
+            {
+                var agg = Agg(lesson.Date);
+                agg.HasLesson = true;
+                if (g.IsOwnClass) agg.IsOwnClassDay = true;
+                agg.Details.Add($"Unterricht: {g.Name}" +
+                    (string.IsNullOrWhiteSpace(lesson.Topic) ? "" : $" – {lesson.Topic}"));
+            }
+
+            foreach (var exam in _exams.GetByGroup(g.Id).Where(e => e.Date >= gridStart && e.Date <= gridEnd))
+            {
+                var agg = Agg(exam.Date);
+                agg.HasExam = true;
+                if (g.IsOwnClass) agg.IsOwnClassDay = true;
+                agg.Details.Add($"Klausur: {exam.Title} ({g.Name})");
+            }
+        }
+
+        for (var i = 0; i < 42; i++)
+        {
+            var date = gridStart.AddDays(i);
+            byDay.TryGetValue(date, out var agg);
+            CalendarDays.Add(new CalendarDayCell(date, date.Month == firstOfMonth.Month, date == today,
+                agg?.HasLesson ?? false, agg?.HasExam ?? false, agg?.IsOwnClassDay ?? false,
+                agg?.Details ?? []));
+        }
+    }
+
+    [RelayCommand]
+    private void PrevMonth()
+    {
+        CalendarMonth = CalendarMonth.AddMonths(-1);
+        OnPropertyChanged(nameof(CalendarMonthLabel));
+        LoadCalendar();
+    }
+
+    [RelayCommand]
+    private void NextMonth()
+    {
+        CalendarMonth = CalendarMonth.AddMonths(1);
+        OnPropertyChanged(nameof(CalendarMonthLabel));
+        LoadCalendar();
+    }
+
+    [RelayCommand]
+    private void CalendarToday()
+    {
+        CalendarMonth = FirstOfMonth(DateTime.Today);
+        OnPropertyChanged(nameof(CalendarMonthLabel));
+        LoadCalendar();
     }
 
     [RelayCommand] private void OpenGroup(GroupChip? c) { if (c is not null) OnNavigateToGroup?.Invoke(c.GroupId); }
     [RelayCommand] private void Refresh() => Load();
+
+    private class DayAgg
+    {
+        public bool HasLesson;
+        public bool HasExam;
+        public bool IsOwnClassDay;
+        public List<string> Details { get; } = [];
+    }
 }
 
 public class LessonItem { public string GroupName { get; set; } = ""; public string Topic { get; set; } = ""; }
 public class TaskItem   { public string Title { get; set; } = ""; public string DueDate { get; set; } = ""; public bool IsOverdue { get; set; } }
 public class GroupChip  { public Guid GroupId { get; set; } public string Name { get; set; } = ""; public string Subject { get; set; } = ""; }
+
+public class CalendarDayCell
+{
+    public int DayNumber { get; }
+    public bool IsCurrentMonth { get; }
+    public bool IsToday { get; }
+    public bool HasLesson { get; }
+    public bool HasExam { get; }
+    public bool IsOwnClassDay { get; }
+    public string Tooltip { get; }
+
+    internal CalendarDayCell(DateOnly date, bool isCurrentMonth, bool isToday,
+        bool hasLesson, bool hasExam, bool isOwnClassDay, List<string> details)
+    {
+        DayNumber      = date.Day;
+        IsCurrentMonth = isCurrentMonth;
+        IsToday        = isToday;
+        HasLesson      = hasLesson;
+        HasExam        = hasExam;
+        IsOwnClassDay  = isOwnClassDay;
+        Tooltip        = details.Count == 0 ? date.ToString("dd.MM.yyyy") : string.Join("\n", details);
+    }
+}