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.

ViewModels/MainViewModel.BrandLogo.cs

@@ -1,4 +1,6 @@
 using System.IO;
+using System.Windows.Media;
+using System.Windows.Media.Imaging;
 using System.Windows.Input;
 using ytLive.Helpers;
 using ytLive.Models;
@@ -22,6 +24,51 @@ public partial class MainViewModel
 
     public string BrandLogoFileDisplay => _layoutStore.GetBrandLogoFileName() ?? "None";
 
+    private ImageSource? _brandLogoPreviewImage;
+    private bool _brandLogoPreviewLoaded;
+
+    /// <summary>The current brand-logo image, for the thumbnail beside the file field in App
+    /// Settings (the creator, 2026-10-30: a myopic user cannot tell what a file *name* looks
+    /// like). Null until an image is set.</summary>
+    public ImageSource? BrandLogoPreviewImage
+    {
+        get
+        {
+            if (!_brandLogoPreviewLoaded) RefreshBrandLogoPreview();
+            return _brandLogoPreviewImage;
+        }
+    }
+
+    internal void RefreshBrandLogoPreview()
+    {
+        _brandLogoPreviewLoaded = true;
+        _brandLogoPreviewImage = LoadBrandLogoImage(_layoutStore.GetBrandLogoAssetId());
+        OnPropertyChanged(nameof(BrandLogoPreviewImage));
+    }
+
+    /// <summary>Decoded once and frozen: the thumbnail and the enlarged preview share it, and a
+    /// frozen bitmap can be handed to any thread without a dispatcher round-trip.</summary>
+    private ImageSource? LoadBrandLogoImage(string? assetId)
+    {
+        if (string.IsNullOrWhiteSpace(assetId)) return null;
+        var bytes = _layoutStore.GetAssetBytes(assetId);
+        if (bytes is not { Length: > 0 }) return null;
+        try
+        {
+            var image = new BitmapImage();
+            image.BeginInit();
+            image.CacheOption = BitmapCacheOption.OnLoad;
+            image.StreamSource = new MemoryStream(bytes);
+            image.EndInit();
+            image.Freeze();
+            return image;
+        }
+        catch (Exception)
+        {
+            return null;   // a corrupt image must not take the dialog down with it
+        }
+    }
+
     private void BrowseBrandLogo()
     {
         var dialog = new Microsoft.Win32.OpenFileDialog
@@ -32,6 +79,7 @@ public partial class MainViewModel
         if (dialog.ShowDialog() != true) return;
         _layoutStore.SetBrandLogo(File.ReadAllBytes(dialog.FileName), Path.GetFileName(dialog.FileName));
         EnsureBrandLogos();
+        RefreshBrandLogoPreview();
         OnPropertyChanged(nameof(BrandLogoFileDisplay));
         ScheduleSave();
     }