Update DisplayAlert method calls to use first window's page

Updated the `DisplayAlert` method calls in `Switch_OnToggled`
and `Toggled` methods across several files to use
`Application.Current!.Windows[0].Page!` instead of
`Application.Current!.MainPage!`. This change ensures alerts
are displayed on the first window's page, accommodating
scenarios with multiple windows or when the main page is not
active. Affected files: `CustomSwitchPage.xaml.cs`,
`PanGestureTestPage.xaml.cs`, `SwitchViewPage.xaml.cs`,
and `SwitchViewModel.cs`.

Demo/App/Pages/CustomSwitchPage.xaml.cs

@@ -17,7 +17,7 @@ async void Switch_OnToggled(object sender, ToggledEventArgs e)
 	{
 		if (_viewModel.EnableEvents)
 		{
-			await Application.Current!.MainPage!.DisplayAlert("Switch toggled (Event)", $"New value: {e.Value}", "OK").ConfigureAwait(false);
+			await Application.Current!.Windows[0].Page!.DisplayAlert("Switch toggled (Event)", $"New value: {e.Value}", "OK").ConfigureAwait(false);
 		}
 	}
 }

Demo/App/Pages/PanGestureTestPage.xaml.cs

@@ -29,6 +29,6 @@ static void CustomSwitch_SwitchPanUpdate(CustomSwitch customSwitch, SwitchPanUpd
 
 	async void Switch_OnToggled(object sender, ToggledEventArgs e)
 	{
-		await Application.Current!.MainPage!.DisplayAlert("Switch toggled (Event)", $"New value: {e.Value}", "OK").ConfigureAwait(false);
+		await Application.Current!.Windows[0].Page!.DisplayAlert("Switch toggled (Event)", $"New value: {e.Value}", "OK").ConfigureAwait(false);
 	}
 }

Demo/App/Pages/SwitchViewPage.xaml.cs

@@ -17,7 +17,7 @@ async void Switch_OnToggled(object sender, ToggledEventArgs e)
 	{
 		if (_viewModel.EnableEvents)
 		{
-			await Application.Current!.MainPage!.DisplayAlert("Switch toggled (Event)", $"New value: {e.Value}", "OK").ConfigureAwait(false);
+			await Application.Current!.Windows[0].Page!.DisplayAlert("Switch toggled (Event)", $"New value: {e.Value}", "OK").ConfigureAwait(false);
 		}
 	}
 }

Demo/App/ViewModels/SwitchViewModel.cs

@@ -13,7 +13,7 @@ public async Task Toggled(bool value)
 	{
 		if (EnableCommands)
 		{
-			await Application.Current!.MainPage!.DisplayAlert("Switch toggled (Command)", $"New value: {value}", "OK").ConfigureAwait(false);
+			await Application.Current!.Windows[0].Page!.DisplayAlert("Switch toggled (Command)", $"New value: {value}", "OK").ConfigureAwait(false);
 		}
 	}
 }