feat(settings): brand-logo thumbnail with a hover enlargement

The creator (2026-10-30): the brand-logo field in App Settings showed only a file NAME, so a
myopic user could not confirm which picture was set. Now a 28x28 thumbnail sits left of the
field showing the actual image, and hovering it opens an enlarged copy (up to 420x320) beside
the panel with the file name underneath. A click collapses it; moving the pointer away closes
it, unless the pointer walked onto the popup itself, where it stays so the creator can look
closely.

Mechanics:
- `MainViewModel.BrandLogoPreviewImage` decodes the stored asset ONCE and freezes it, so the
  thumbnail and the enlargement share one bitmap and no dispatcher hop is needed. A corrupt
  image returns null instead of taking the dialog down.
- The thumbnail is hidden when no image is set (NotNullToVis), so there is nothing to hover and
  no empty frame can appear.
- The popup is a separate window: WPF refuses to open one until its placement target is in a
  live visual tree, which is why the test hosts the control in a real window rather than
  asserting on a detached control.

Guard: BrandLogoPreviewTests — the wiring (image binding, the no-image gate, the four gesture
handlers, the readable enlargement size) and the control's open/close behaviour.

Controls/OverlayHost.xaml.cs

@@ -11,6 +11,36 @@ public partial class OverlayHost : UserControl
         InitializeComponent();
     }
 
+    /// <summary>Is the enlarged brand-logo preview showing? Test seam — a Popup is a separate
+    /// window, so nothing else can observe it.</summary>
+    internal bool IsBrandLogoPreviewOpen => BrandLogoPreviewPopup.IsOpen;
+
+    /// <summary>Hover opens the enlargement. Nothing to show when no image is set, so the
+    /// thumbnail simply does nothing in that case.</summary>
+    private void BrandLogoThumb_MouseEnter(object sender, System.Windows.Input.MouseEventArgs e)
+        => OpenBrandLogoPreview();
+
+    /// <summary>Leaving the thumbnail closes it — unless the pointer walked onto the popup
+    /// itself (which sits beside it), where it stays put so the creator can look closely.</summary>
+    private void BrandLogoThumb_MouseLeave(object sender, System.Windows.Input.MouseEventArgs e)
+    {
+        if (BrandLogoPreviewPopup.Child?.IsMouseOver == true) return;
+        CloseBrandLogoPreview();
+    }
+
+    /// <summary>A click collapses the enlargement — the explicit "enough, thanks" gesture.</summary>
+    private void BrandLogoThumb_Click(object sender, System.Windows.Input.MouseButtonEventArgs e)
+        => CloseBrandLogoPreview();
+
+    private void BrandLogoPreviewPopup_MouseLeave(object sender, System.Windows.Input.MouseEventArgs e)
+        => CloseBrandLogoPreview();
+
+    /// <summary>The thumbnail is hidden when no image is set (see the XAML), so there is nothing
+    /// to open in that case — no empty frame can appear.</summary>
+    internal void OpenBrandLogoPreview() => BrandLogoPreviewPopup.IsOpen = true;
+
+    internal void CloseBrandLogoPreview() => BrandLogoPreviewPopup.IsOpen = false;
+
     private void BackToAbout_Click(object sender, RoutedEventArgs e)
     {
         if (DataContext is MainViewModel vm)