refactor: extract OverlayHost (settings/bug/feature/about dialogs) — Phase 2, Commit 18

OverlayHost.xaml(.cs) takes the window-root overlay dialog grid (settings/
bug/feature/about panels); BackToAbout_Click moved in via inherited DataContext.
WebViewHostPanel + ToastArea stay as thin window-root stubs so the WebView2 ctor
init and NotificationService InArea('ToastArea') name lookups are untouched.
This completes Phase 2: MainWindow.xaml is now a 123-line shell.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 2 tracker updated in same commit.

MainWindow.xaml.cs

@@ -135,15 +135,6 @@ public partial class MainWindow : Window
             : null;
     }
 
-    private void BackToAbout_Click(object sender, RoutedEventArgs e)
-    {
-        if (DataContext is ViewModels.MainViewModel vm)
-        {
-            vm.IsLicensesOpen = false;
-            vm.IsLicenseEntryOpen = false;
-        }
-    }
-
     private void Window_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
     {
         if (e.OriginalSource is not DependencyObject original) return;