feat: sync guard - Blockieren alter Clients beim sync

LehrerApp.Desktop/Views/MainWindow.axaml

@@ -30,8 +30,7 @@
   <DrawerPage x:Name="RootDrawer"
               DrawerLength="220"
               DrawerBehavior="Auto"
-              DrawerLayoutBehavior="CompactInline"
-              Content="{Binding CurrentPage}">
+              DrawerLayoutBehavior="CompactInline">
 
     <DrawerPage.DataTemplates>
       <DataTemplate DataType="vm:DashboardViewModel">
@@ -63,6 +62,26 @@
       </DataTemplate>
     </DrawerPage.DataTemplates>
 
+    <DrawerPage.Content>
+      <DockPanel>
+        <Border DockPanel.Dock="Top"
+                IsVisible="{Binding SyncStatus.IsIncompatibleVersion}"
+                Background="#B3261E" Padding="16,12">
+          <StackPanel Orientation="Horizontal" Spacing="10">
+            <TextBlock Text="⚠" FontSize="18" Foreground="White"
+                       VerticalAlignment="Center"/>
+            <StackPanel>
+              <TextBlock Text="Software-Update erforderlich"
+                         Foreground="White" FontWeight="SemiBold"/>
+              <TextBlock Text="{x:Static vm:SyncStatusViewModel.IncompatibleVersionMessage}"
+                         Foreground="White" Opacity="0.92" TextWrapping="Wrap"/>
+            </StackPanel>
+          </StackPanel>
+        </Border>
+        <ContentControl Content="{Binding CurrentPage}"/>
+      </DockPanel>
+    </DrawerPage.Content>
+
     <DrawerPage.Drawer>
       <DockPanel Classes.compact="{Binding !#RootDrawer.IsOpen}">
         <DockPanel.Styles>

LehrerApp.Desktop/Views/SyncStatusBar.axaml

@@ -4,14 +4,19 @@
              x:Class="LehrerApp.Desktop.Views.SyncStatusBar"
              x:DataType="vm:SyncStatusViewModel">
   <Grid ColumnDefinitions="*,Auto">
-    <StackPanel Grid.Column="0">
+    <StackPanel Grid.Column="0" IsVisible="{Binding !IsIncompatibleVersion}">
       <TextBlock Text="{Binding StatusText}" FontSize="12" Opacity="0.7"
                  TextTrimming="CharacterEllipsis"/>
       <TextBlock Text="{Binding LastSyncText}" FontSize="10" Opacity="0.4"/>
     </StackPanel>
+    <StackPanel Grid.Column="0" IsVisible="{Binding IsIncompatibleVersion}">
+      <TextBlock Text="⚠ Update erforderlich" FontSize="12" FontWeight="SemiBold"
+                 Foreground="#D32F2F"/>
+      <TextBlock Text="Sync angehalten" FontSize="10" Foreground="#D32F2F" Opacity="0.8"/>
+    </StackPanel>
     <Button Grid.Column="1" Content="↻" FontSize="14"
             Command="{Binding SyncNowCommand}"
-            IsVisible="{Binding IsServerConfigured}"
+            IsVisible="{Binding CanAttemptSync}"
             Background="Transparent" Padding="6,4"
             ToolTip.Tip="Jetzt synchronisieren"/>
   </Grid>