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"
             x:Class="LehrerApp.Desktop.Views.Groups.GroupDetailView"
             x:DataType="vm:GroupDetailViewModel">

  <Grid RowDefinitions="Auto,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 FontSize="12" Opacity="0.5">
            <Run Text="{Binding StudentCount}"/>
            <Run Text="Schüler"/>
          </TextBlock>
        </StackPanel>
        <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="8">
          <Button Content="+ Schüler"
                  Command="{Binding AddStudentCommand}"/>
          <Button Content="+ Klausur"
                  Command="{Binding AddExamCommand}"/>
        </StackPanel>
      </Grid>
    </Border>

    <!-- ── Tab-Leiste ──────────────────────────────────────────────────── -->
    <Border Grid.Row="1"
            BorderBrush="{DynamicResource SystemControlForegroundBaseLowBrush}"
            BorderThickness="0,0,0,1"
            Padding="16,0">
      <StackPanel Orientation="Horizontal" Spacing="4">
        <Button Content="Übersicht"
                Command="{Binding SwitchTabCommand}"
                CommandParameter="{x:Static vm:GroupTab.Overview}"
                Classes.active="{Binding ActiveTab,
                  Converter={x:Static ObjectConverters.Equal},
                  ConverterParameter={x:Static vm:GroupTab.Overview}}"
                Background="Transparent" Padding="12,8"/>
        <Button Content="Schüler"
                Command="{Binding SwitchTabCommand}"
                CommandParameter="{x:Static vm:GroupTab.Students}"
                Classes.active="{Binding ActiveTab,
                  Converter={x:Static ObjectConverters.Equal},
                  ConverterParameter={x:Static vm:GroupTab.Students}}"
                Background="Transparent" Padding="12,8"/>
        <Button Content="Klausuren"
                Command="{Binding SwitchTabCommand}"
                CommandParameter="{x:Static vm:GroupTab.Exams}"
                Classes.active="{Binding ActiveTab,
                  Converter={x:Static ObjectConverters.Equal},
                  ConverterParameter={x:Static vm:GroupTab.Exams}}"
                Background="Transparent" Padding="12,8"/>
        <Button Content="Noten"
                Command="{Binding SwitchTabCommand}"
                CommandParameter="{x:Static vm:GroupTab.Grades}"
                Background="Transparent" Padding="12,8"/>
        <Button Content="Planung"
                Command="{Binding SwitchTabCommand}"
                CommandParameter="{x:Static vm:GroupTab.Planner}"
                Background="Transparent" Padding="12,8"/>
        <Button Content="Dokumentation"
                Command="{Binding SwitchTabCommand}"
                CommandParameter="{x:Static vm:GroupTab.Documentation}"
                Background="Transparent" Padding="12,8"/>
      </StackPanel>
    </Border>

    <!-- ── Tab-Inhalt ──────────────────────────────────────────────────── -->
    <ScrollViewer Grid.Row="2" Padding="20">

      <!-- Schülerliste -->
      <DataGrid ItemsSource="{Binding Students}"
                IsVisible="{Binding ActiveTab,
                  Converter={x:Static ObjectConverters.Equal},
                  ConverterParameter={x:Static vm:GroupTab.Students}}"
                AutoGenerateColumns="False"
                IsReadOnly="True"
                GridLinesVisibility="Horizontal"
                CanUserReorderColumns="False"
                CanUserResizeColumns="True">
        <DataGrid.Columns>
          <DataGridTextColumn Header="Name"
                              Binding="{Binding FullName}"
                              Width="*"/>
        </DataGrid.Columns>
      </DataGrid>

      <!-- Klausuren -->
      <DataGrid ItemsSource="{Binding Exams}"
                IsVisible="{Binding ActiveTab,
                  Converter={x:Static ObjectConverters.Equal},
                  ConverterParameter={x:Static vm:GroupTab.Exams}}"
                AutoGenerateColumns="False"
                IsReadOnly="True"
                GridLinesVisibility="Horizontal"
                CanUserReorderColumns="False">
        <DataGrid.Columns>
          <DataGridTextColumn Header="Datum"
                              Binding="{Binding Date}"
                              Width="100"/>
          <DataGridTextColumn Header="Titel"
                              Binding="{Binding Title}"
                              Width="*"/>
          <DataGridTextColumn Header="Status"
                              Binding="{Binding Status}"
                              Width="120"/>
        </DataGrid.Columns>
      </DataGrid>

      <!-- Übersicht (Platzhalter) -->
      <StackPanel IsVisible="{Binding ActiveTab,
                    Converter={x:Static ObjectConverters.Equal},
                    ConverterParameter={x:Static vm:GroupTab.Overview}}"
                  Spacing="12">
        <TextBlock Text="Übersicht" FontSize="16" FontWeight="SemiBold"/>
        <TextBlock Text="Hier erscheint eine Zusammenfassung der Lerngruppe."
                   Opacity="0.5"/>
      </StackPanel>

    </ScrollViewer>

  </Grid>

</UserControl>