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/Views/MainWindow.axaml

@@ -21,10 +21,10 @@
     darunter als Overlay-Drawer mit Hamburger-Button (automatisch).
     Kein eigener Code nötig.
   -->
-  <DrawerPage DrawerLength="220"
+  <DrawerPage x:Name="RootDrawer"
+              DrawerLength="220"
               DrawerBehavior="Auto"
               DrawerLayoutBehavior="CompactInline"
-              
               Content="{Binding CurrentPage}">
 
     <DrawerPage.DataTemplates>
@@ -52,15 +52,48 @@
     </DrawerPage.DataTemplates>
 
     <DrawerPage.Drawer>
-      <DockPanel>
+      <DockPanel Classes.compact="{Binding !#RootDrawer.IsOpen}">
+        <DockPanel.Styles>
+          <!-- Basisgröße der Icons als Style (nicht lokal), damit der Compact-Selector sie
+               überschreiben kann – lokale Werte hätten immer Vorrang vor Style-Settern. -->
+          <Style Selector="TextBlock.navicon">
+            <Setter Property="FontSize" Value="15"/>
+          </Style>
+          <!-- Minimierter Zustand: Icons größer & zentriert, Beschriftungen ausgeblendet -->
+          <Style Selector="DockPanel.compact TextBlock.navlabel">
+            <Setter Property="IsVisible" Value="False"/>
+          </Style>
+          <Style Selector="DockPanel.compact TextBlock.navicon">
+            <Setter Property="FontSize" Value="20"/>
+          </Style>
+          <Style Selector="DockPanel.compact Button.navitem">
+            <Setter Property="HorizontalContentAlignment" Value="Center"/>
+          </Style>
+          <Style Selector="DockPanel.compact TextBlock.navsection">
+            <Setter Property="IsVisible" Value="False"/>
+          </Style>
+          <Style Selector="DockPanel.compact TextBlock.appsubtitle">
+            <Setter Property="IsVisible" Value="False"/>
+          </Style>
+          <Style Selector="DockPanel.compact Border.appheader">
+            <Setter Property="HorizontalAlignment" Value="Center"/>
+          </Style>
+        </DockPanel.Styles>
 
         <Border DockPanel.Dock="Top" Padding="16,20,16,14"
                 BorderBrush="{DynamicResource SystemControlForegroundBaseLowBrush}"
                 BorderThickness="0,0,0,1">
-          <StackPanel>
-            <TextBlock Text="LehrerApp" FontSize="20" FontWeight="SemiBold"/>
-            <TextBlock Text="{Binding CurrentSchoolYear}"
-                       FontSize="12" Opacity="0.55" Margin="0,2,0,0"/>
+          <StackPanel Orientation="Horizontal" Spacing="10">
+            <Border Classes="appheader" Width="32" Height="32" CornerRadius="8"
+                    Background="{DynamicResource SystemControlBackgroundAccentBrush}">
+              <TextBlock Text="L" FontSize="16" FontWeight="Bold" Foreground="White"
+                         HorizontalAlignment="Center" VerticalAlignment="Center"/>
+            </Border>
+            <StackPanel Classes="appsubtitle" VerticalAlignment="Center">
+              <TextBlock Text="LehrerApp" FontSize="15" FontWeight="SemiBold"/>
+              <TextBlock Text="{Binding CurrentSchoolYear}"
+                         FontSize="11" Opacity="0.55"/>
+            </StackPanel>
           </StackPanel>
         </Border>
 
@@ -72,49 +105,84 @@
 
         <ScrollViewer>
           <StackPanel Margin="8,12,8,0" Spacing="2">
-            <Button Content="📊  Dashboard" HorizontalAlignment="Stretch"
+            <Button Classes="navitem" HorizontalAlignment="Stretch"
                     HorizontalContentAlignment="Left" Background="Transparent"
                     Padding="10,8" CornerRadius="6"
                     Command="{Binding NavigateToCommand}"
-                    CommandParameter="{x:Static vm:NavItem.Dashboard}"/>
+                    CommandParameter="{x:Static vm:NavItem.Dashboard}">
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <TextBlock Classes="navicon" Text="📊" Width="20" TextAlignment="Center"/>
+                <TextBlock Classes="navlabel" Text="Dashboard"/>
+              </StackPanel>
+            </Button>
 
-            <TextBlock Text="UNTERRICHT" FontSize="10" FontWeight="Bold"
+            <TextBlock Classes="navsection" Text="UNTERRICHT" FontSize="10" FontWeight="Bold"
                        Opacity="0.4" Margin="10,14,0,4"/>
 
-            <Button Content="🏫  Lerngruppen" HorizontalAlignment="Stretch"
+            <Button Classes="navitem" HorizontalAlignment="Stretch"
                     HorizontalContentAlignment="Left" Background="Transparent"
                     Padding="10,8" CornerRadius="6"
                     Command="{Binding NavigateToCommand}"
-                    CommandParameter="{x:Static vm:NavItem.Groups}"/>
-            <Button Content="👤  Schüler" HorizontalAlignment="Stretch"
+                    CommandParameter="{x:Static vm:NavItem.Groups}">
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <TextBlock Classes="navicon" Text="🏫" Width="20" TextAlignment="Center"/>
+                <TextBlock Classes="navlabel" Text="Lerngruppen"/>
+              </StackPanel>
+            </Button>
+            <Button Classes="navitem" HorizontalAlignment="Stretch"
                     HorizontalContentAlignment="Left" Background="Transparent"
                     Padding="10,8" CornerRadius="6"
                     Command="{Binding NavigateToCommand}"
-                    CommandParameter="{x:Static vm:NavItem.Students}"/>
-            <Button Content="📝  Klausuren" HorizontalAlignment="Stretch"
+                    CommandParameter="{x:Static vm:NavItem.Students}">
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <TextBlock Classes="navicon" Text="👤" Width="20" TextAlignment="Center"/>
+                <TextBlock Classes="navlabel" Text="Schüler"/>
+              </StackPanel>
+            </Button>
+            <Button Classes="navitem" HorizontalAlignment="Stretch"
                     HorizontalContentAlignment="Left" Background="Transparent"
                     Padding="10,8" CornerRadius="6"
                     Command="{Binding NavigateToCommand}"
-                    CommandParameter="{x:Static vm:NavItem.Exams}"/>
-            <Button Content="📅  Planung" HorizontalAlignment="Stretch"
+                    CommandParameter="{x:Static vm:NavItem.Exams}">
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <TextBlock Classes="navicon" Text="📝" Width="20" TextAlignment="Center"/>
+                <TextBlock Classes="navlabel" Text="Klausuren"/>
+              </StackPanel>
+            </Button>
+            <Button Classes="navitem" HorizontalAlignment="Stretch"
                     HorizontalContentAlignment="Left" Background="Transparent"
                     Padding="10,8" CornerRadius="6"
                     Command="{Binding NavigateToCommand}"
-                    CommandParameter="{x:Static vm:NavItem.Planner}"/>
+                    CommandParameter="{x:Static vm:NavItem.Planner}">
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <TextBlock Classes="navicon" Text="📅" Width="20" TextAlignment="Center"/>
+                <TextBlock Classes="navlabel" Text="Planung"/>
+              </StackPanel>
+            </Button>
 
-            <TextBlock Text="VERWALTUNG" FontSize="10" FontWeight="Bold"
+            <TextBlock Classes="navsection" Text="VERWALTUNG" FontSize="10" FontWeight="Bold"
                        Opacity="0.4" Margin="10,14,0,4"/>
 
-            <Button Content="⏱  Arbeitszeit" HorizontalAlignment="Stretch"
+            <Button Classes="navitem" HorizontalAlignment="Stretch"
                     HorizontalContentAlignment="Left" Background="Transparent"
                     Padding="10,8" CornerRadius="6"
                     Command="{Binding NavigateToCommand}"
-                    CommandParameter="{x:Static vm:NavItem.Workload}"/>
-            <Button Content="⚙️  Einstellungen" HorizontalAlignment="Stretch"
+                    CommandParameter="{x:Static vm:NavItem.Workload}">
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <TextBlock Classes="navicon" Text="⏱" Width="20" TextAlignment="Center"/>
+                <TextBlock Classes="navlabel" Text="Arbeitszeit"/>
+              </StackPanel>
+            </Button>
+            <Button Classes="navitem" HorizontalAlignment="Stretch"
                     HorizontalContentAlignment="Left" Background="Transparent"
                     Padding="10,8" CornerRadius="6"
                     Command="{Binding NavigateToCommand}"
-                    CommandParameter="{x:Static vm:NavItem.Settings}"/>
+                    CommandParameter="{x:Static vm:NavItem.Settings}">
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <TextBlock Classes="navicon" Text="⚙️" Width="20" TextAlignment="Center"/>
+                <TextBlock Classes="navlabel" Text="Einstellungen"/>
+              </StackPanel>
+            </Button>
           </StackPanel>
         </ScrollViewer>
       </DockPanel>