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/WindowDialogService.cs

@@ -143,7 +143,8 @@ public sealed class WindowDialogService : IDialogService
             diffVm.CloseAction = () => diffDlg.Close();
             diffDlg.Closed += (_, _) => _diffViewerWindow = null;
             _diffViewerWindow = diffDlg;
-            diffDlg.Show(_owner);
+            diffDlg.CenterOn(_owner);
+            diffDlg.Show();   // no owner: an owned window would be pinned above the main window
         };
         vm.ConfirmAction = message => ConfirmAsync(message);
         if (Shell is { } shell)