refactor(ui): redesign list settings and merge modals with custom chrome

Both modals now use SystemDecorations=None with a draggable title bar,
sectioned layout matching the rest of the island shell, Escape-to-cancel,
and themed brushes instead of hard-coded colours. ListSettings adds a
Browse... button that reads agent frontmatter from arbitrary .md files.

src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml.cs

@@ -1,4 +1,5 @@
 using Avalonia.Controls;
+using Avalonia.Input;
 using ClaudeDo.Ui.ViewModels.Modals;
 
 namespace ClaudeDo.Ui.Views.Modals;
@@ -16,4 +17,10 @@ public partial class MergeModalView : Window
             }
         };
     }
+
+    private void TitleBar_PointerPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+            BeginMoveDrag(e);
+    }
 }