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.

Controls/OverlayHost.xaml.cs

@@ -0,0 +1,22 @@
+using System.Windows;
+using System.Windows.Controls;
+using ytLive.ViewModels;
+
+namespace ytLive.Controls;
+
+public partial class OverlayHost : UserControl
+{
+    public OverlayHost()
+    {
+        InitializeComponent();
+    }
+
+    private void BackToAbout_Click(object sender, RoutedEventArgs e)
+    {
+        if (DataContext is MainViewModel vm)
+        {
+            vm.IsLicensesOpen = false;
+            vm.IsLicenseEntryOpen = false;
+        }
+    }
+}