Fix: URI text box no longer shows under webcam source

The web source section's visibility was bound to SelectedElement.Type
via a converter, but WebcamSceneConfig has no Type property — the WPF
binding path didn't resolve correctly and the section leaked through.

Added IsWebSource virtual property to SceneElement (mirrors existing
IsWebcam pattern), overridden in Source to return
Type == SourceType.WebSource. Section visibility now binds directly to
SelectedElement.IsWebSource via BoolToVis.

1c of 7-issue polish batch. 207 tests, 0 warnings.

MainWindow.xaml

@@ -449,7 +449,7 @@
                             </StackPanel>
 
                             <!-- Web source config -->
-                            <StackPanel Visibility="{Binding SelectedElement.Type, Converter={StaticResource SourceTypeToWebVis}, ConverterParameter=WebSource}" Margin="0,0,0,8">
+                            <StackPanel Visibility="{Binding SelectedElement.IsWebSource, Converter={StaticResource BoolToVis}}" Margin="0,0,0,8">
                                 <TextBlock Text="URI" Foreground="#a0a0b0" FontSize="11" Margin="0,0,0,2"/>
                                 <DockPanel>
                                     <Button DockPanel.Dock="Right" Style="{StaticResource IconButton}"