fix(ui): open diff viewer and merge window as independent top-level windows

Owned windows are pinned above their owner by Windows, so the main window
could never come to the front. Show() without an owner and centre manually.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs

@@ -161,7 +161,8 @@ public partial class DetailsIslandView : UserControl
                 var window = new DiffViewerView { DataContext = diffVm };
                 window.Closed += (_, _) => _diffViewerWindow = null;
                 _diffViewerWindow = window;
-                window.Show(owner);
+                window.CenterOn(owner);
+                window.Show();   // no owner: an owned window would be pinned above the main window
                 return System.Threading.Tasks.Task.CompletedTask;
             };