LehrerApp/LehrerApp.Desktop/Views/Groups/GroupDetailView.axaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="clr-namespace:LehrerApp.Desktop.ViewModels.Groups"
             xmlns:views="clr-namespace:LehrerApp.Desktop.Views.Groups"
             xmlns:models="clr-namespace:LehrerApp.Core.Models;assembly=LehrerApp.Core"
             x:Class="LehrerApp.Desktop.Views.Groups.GroupDetailView"
             x:DataType="vm:GroupDetailViewModel">

  <Grid RowDefinitions="Auto,*">

    <!-- Header -->
    <Border Grid.Row="0" Padding="20,16"
            BorderBrush="{DynamicResource SystemControlForegroundBaseLowBrush}"
            BorderThickness="0,0,0,1">
      <Grid ColumnDefinitions="*,Auto">
        <StackPanel Grid.Column="0">
          <TextBlock Text="{Binding GroupTitle}" FontSize="22" FontWeight="SemiBold"/>
          <TextBlock Text="{Binding GroupSubtitle}" FontSize="12" Opacity="0.5"/>
        </StackPanel>
        <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="8">
          <TextBlock VerticalAlignment="Center" Opacity="0.6" FontSize="13">
            <Run Text="{Binding StudentCount}"/>
            <Run Text=" Schüler"/>
          </TextBlock>
          <Button Content="＋ Schüler"   Command="{Binding AddStudentCommand}"/>
          <Button Content="− Austragen"  Command="{Binding RemoveStudentCommand}"
                  IsVisible="{Binding SelectedStudent, Converter={x:Static ObjectConverters.IsNotNull}}"/>
          <Button Content="＋ Klausur"   Command="{Binding AddExamCommand}"/>
        </StackPanel>
      </Grid>
    </Border>

    <TabbedPage Grid.Row="1" TabPlacement="Top" SelectedIndex="{Binding ActiveTabIndex}">

      <!-- Tab: Übersicht -->
      <ContentPage Header="Übersicht">
        <ScrollViewer Padding="20">
          <StackPanel Spacing="12">
            <TextBlock Text="{Binding GroupSubtitle}" Opacity="0.7" FontSize="14"/>
            <TextBlock Text="Hier erscheint später eine Zusammenfassung der Lerngruppe."
                       Opacity="0.4"/>
          </StackPanel>
        </ScrollViewer>
      </ContentPage>

      <!-- Tab: Schüler -->
      <ContentPage Header="Schüler">
        <DataGrid ItemsSource="{Binding Students}"
                  SelectedItem="{Binding SelectedStudent}"
                  AutoGenerateColumns="False"
                  IsReadOnly="True"
                  GridLinesVisibility="Horizontal"
                  CanUserReorderColumns="False"
                  CanUserResizeColumns="True"
                  Margin="0">
          <DataGrid.Columns>
            <DataGridTextColumn Header="Name"    Binding="{Binding FullName}"    Width="*"/>
            <DataGridTextColumn Header="Zeitraum" Binding="{Binding PeriodLabel}" Width="90"/>
          </DataGrid.Columns>
        </DataGrid>
      </ContentPage>

      <!-- Tab: Mitarbeit -->
      <ContentPage Header="Mitarbeit">
        <views:ParticipationTabView DataContext="{Binding ParticipationTab}"/>
      </ContentPage>

      <!-- Tab: Klausuren -->
      <ContentPage Header="Klausuren">
        <Grid RowDefinitions="Auto,*">
          <StackPanel Grid.Row="0" Orientation="Horizontal" Spacing="8" Margin="0,0,0,8"
                      IsVisible="{Binding SelectedExam, Converter={x:Static ObjectConverters.IsNotNull}}">
            <Button Content="Bearbeiten"    Command="{Binding EditExamCommand}"/>
            <Button Content="Duplizieren"   Command="{Binding DuplicateExamCommand}"/>
            <SplitButton Content="Status ▸" Command="{Binding AdvanceExamStatusCommand}">
              <SplitButton.Flyout>
                <MenuFlyout Placement="BottomEdgeAlignedLeft">
                  <MenuItem Header="Geplant"       Command="{Binding SetExamStatusCommand}"
                            CommandParameter="{x:Static models:ExamStatus.Planned}"/>
                  <MenuItem Header="Durchgeführt"  Command="{Binding SetExamStatusCommand}"
                            CommandParameter="{x:Static models:ExamStatus.Conducted}"/>
                  <MenuItem Header="Korrigiert"    Command="{Binding SetExamStatusCommand}"
                            CommandParameter="{x:Static models:ExamStatus.Graded}"/>
                  <MenuItem Header="Zurückgegeben" Command="{Binding SetExamStatusCommand}"
                            CommandParameter="{x:Static models:ExamStatus.Returned}"/>
                </MenuFlyout>
              </SplitButton.Flyout>
            </SplitButton>
            <Button Content="Löschen"       Command="{Binding DeleteExamCommand}"/>
          </StackPanel>
          <DataGrid Grid.Row="1" ItemsSource="{Binding Exams}"
                    SelectedItem="{Binding SelectedExam}"
                    AutoGenerateColumns="False"
                    IsReadOnly="True"
                    GridLinesVisibility="Horizontal"
                    CanUserReorderColumns="False">
            <DataGrid.Columns>
              <DataGridTextColumn Header="Datum"  Binding="{Binding Date}"  Width="110"/>
              <DataGridTextColumn Header="Titel"  Binding="{Binding Title}" Width="*"/>
              <DataGridTemplateColumn Header="Status" Width="150">
                <DataGridTemplateColumn.CellTemplate>
                  <DataTemplate x:DataType="vm:ExamSummary">
                    <Border Background="{Binding StatusColorHex}" CornerRadius="4"
                            Padding="8,2" HorizontalAlignment="Left">
                      <TextBlock Text="{Binding StatusLabel}" FontSize="12" Foreground="White"/>
                    </Border>
                  </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
              </DataGridTemplateColumn>
              <DataGridTextColumn Header="Rückgabe" Binding="{Binding ReturnedAtDisplay}" Width="100"/>
            </DataGrid.Columns>
            <DataGrid.ContextMenu>
              <ContextMenu>
                <MenuItem Header="Bearbeiten"    Command="{Binding EditExamCommand}"/>
                <MenuItem Header="Duplizieren"   Command="{Binding DuplicateExamCommand}"/>
                <MenuItem Header="Status">
                  <MenuItem Header="Weiter" Command="{Binding AdvanceExamStatusCommand}"/>
                  <Separator/>
                  <MenuItem Header="Geplant"       Command="{Binding SetExamStatusCommand}"
                            CommandParameter="{x:Static models:ExamStatus.Planned}"/>
                  <MenuItem Header="Durchgeführt"  Command="{Binding SetExamStatusCommand}"
                            CommandParameter="{x:Static models:ExamStatus.Conducted}"/>
                  <MenuItem Header="Korrigiert"    Command="{Binding SetExamStatusCommand}"
                            CommandParameter="{x:Static models:ExamStatus.Graded}"/>
                  <MenuItem Header="Zurückgegeben" Command="{Binding SetExamStatusCommand}"
                            CommandParameter="{x:Static models:ExamStatus.Returned}"/>
                </MenuItem>
                <Separator/>
                <MenuItem Header="Löschen"       Command="{Binding DeleteExamCommand}"/>
              </ContextMenu>
            </DataGrid.ContextMenu>
          </DataGrid>
        </Grid>
      </ContentPage>

      <!-- Tab: Noten -->
      <ContentPage Header="Noten">
        <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
          <TextBlock Text="Notenübersicht" FontSize="16" Opacity="0.4"
                     HorizontalAlignment="Center"/>
          <TextBlock Text="Wird implementiert." FontSize="12" Opacity="0.3"
                     HorizontalAlignment="Center"/>
        </StackPanel>
      </ContentPage>

      <!-- Tab: Planung -->
      <ContentPage Header="Planung">
        <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
          <TextBlock Text="Unterrichtseinheiten" FontSize="16" Opacity="0.4"
                     HorizontalAlignment="Center"/>
          <TextBlock Text="Wird implementiert." FontSize="12" Opacity="0.3"
                     HorizontalAlignment="Center"/>
        </StackPanel>
      </ContentPage>

      <!-- Tab: Dokumentation -->
      <ContentPage Header="Dokumentation">
        <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
          <TextBlock Text="Schülerdokumentation" FontSize="16" Opacity="0.4"
                     HorizontalAlignment="Center"/>
          <TextBlock Text="Wird implementiert." FontSize="12" Opacity="0.3"
                     HorizontalAlignment="Center"/>
        </StackPanel>
      </ContentPage>

    </TabbedPage>
  </Grid>
</UserControl>