Some window managers (on both X11 and Wayland) hide the title-bar close
button for Gtk.Dialog windows that have no action buttons. The previous
fix (PR #333) added set_decorated(True) which was not sufficient.
This adds an explicit Close action button to the dialog so users always
have a visible way to dismiss it, regardless of WM behavior. The
instant-apply pattern is preserved — settings still apply immediately
on change.