fix(checkin): align modal overlay with reference backdrop style

Reference .modal-backdrop is plain black at 50% opacity with no blur;
replace the runtime overlay's blue-dark tint and backdrop blur to
match it 1:1.

assets/css/frontend/modal-runtime.css

@@ -62,8 +62,8 @@ body.aksrt-modal-runtime-open {
 .aksrt-modal-runtime__overlay {
   position: absolute;
   inset: 0;
-  background: rgba(15, 18, 28, 0.48);
-  backdrop-filter: blur(2px);
+  /* 参考主题 .modal-backdrop 同款：纯黑 50%，无模糊、无色调偏移 */
+  background: rgba(0, 0, 0, 0.5);
 }
 
 .aksrt-modal-runtime__dialog {