🐛(frontend) fix modal backdrop z-index

Recent upgrade of ui-kit removed the z-index for
the modal backdrop, causing it to be hidden behind
other elements. This commit restores the z-index
 to ensure the modal backdrop is displayed correctly.

Author: AntoLC

src/frontend/apps/impress/src/cunningham/cunningham-style.css

@@ -38,6 +38,13 @@
   );
 }
 
+/**
+ * Modal
+*/
+.c__modal__backdrop {
+  z-index: 1000;
+}
+
 /**
  * Tooltip
 */