fix: Add missing using statement for SystemNavigationManager

Added explicit 'using Windows.UI.Core;' to ResponseNavigator.cs to resolve
build error. SystemNavigationManager is in the Windows.UI.Core namespace,
which is only included globally for non-WINUI platforms.

Co-authored-by: kazo0 <4793020+kazo0@users.noreply.github.com>

Author: Copilot

src/Uno.Extensions.Navigation.UI/ResponseNavigator.cs

@@ -1,4 +1,6 @@
-namespace Uno.Extensions.Navigation;
+using Windows.UI.Core;
+
+namespace Uno.Extensions.Navigation;
 
 public class ResponseNavigator<TResult> : IResponseNavigator, IInstance<IServiceProvider>
 {