fix(checkin): restore fullscreen reference backdrop on details modal

The reference modal backdrop is a fullscreen, non-transparent black
50% overlay; drop the mistaken transparent-overlay override for the
checkin shell and keep the shared runtime backdrop.

assets/css/frontend/modal-runtime.css

@@ -236,12 +236,6 @@ body.aksrt-modal-runtime-open {
   padding: 0;
 }
 
-/* 参考站弹窗打开时页面内容保持完全可见（无压暗蒙版）；
-   overlay 仅承担「点击空白处关闭」的热区。 */
-.aksrt-modal-runtime.aksrt-checkin-details-modal .aksrt-modal-runtime__overlay {
-  background: transparent;
-}
-
 .aksrt-modal-runtime.aksrt-checkin-details-modal .aksrt-modal-runtime__skeleton {
   padding: 24px 18px;
 }