fix(diff): install background renderers on attach so themed brushes resolve

src/ClaudeDo.Ui/Views/Controls/DiffTextView.axaml.cs

@@ -69,17 +69,31 @@ public partial class DiffTextView : UserControl
     private ScrollViewer? _leftScroll, _rightScroll;
     private bool _scrollHooked;
     private bool _syncing;
+    private bool _renderersInstalled;
 
     public DiffTextView()
     {
         InitializeComponent();
         _leftTm = LeftEditor.InstallTextMate(Registry);
         _rightTm = RightEditor.InstallTextMate(Registry);
-        InstallRenderers();
         ReloadFile();
         HookScrollSync();
     }
 
+    /// Brushes and the mono typeface only resolve once the control is in the visual tree —
+    /// a detached control has no resource parent to walk up to, and its styles haven't been
+    /// applied yet. Installing the renderers in the constructor would freeze them on their
+    /// hardcoded fallbacks for good, since they capture their brushes once.
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        if (_renderersInstalled) return;
+        _renderersInstalled = true;
+        InstallRenderers();
+        RebuildMargins();
+        InvalidateRenderers();
+    }
+
     protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
     {
         base.OnPropertyChanged(change);