Webcam menu gates on the app-wide identity (TASK 26)

MainWindow.xaml

@@ -276,7 +276,7 @@
                             <Button.ContextMenu>
                                 <ContextMenu DataContext="{Binding PlacementTarget.DataContext, RelativeSource={RelativeSource Self}}">
                                     <MenuItem Header="Webcam" Command="{Binding AddWebcamCommand}"
-                                              IsEnabled="{Binding CanAddWebcamToStagedScene}"
+                                              IsEnabled="{Binding CanAddWebcam}"
                                               ToolTip="One webcam at a time — it's already in your stream"/>
                                     <MenuItem Header="Image" Command="{Binding AddImageCommand}"/>
                                     <MenuItem Header="Text" Command="{Binding AddSourceCommand}" CommandParameter="{x:Static models:SourceType.TextOverlay}"/>