fix: Scrollbars on the thumbnails to be auto visible (#2605)

The thumbnail view had hte scrollbars not visible and they didn't
appear, and that made it hard to discover how to scroll.

platform/ui/src/components/ThumbnailList/ThumbnailList.jsx

@@ -11,7 +11,7 @@ const ThumbnailList = ({
   onClickUntrack,
 }) => {
   return (
-    <div className="py-3 bg-black">
+    <div className="py-3 bg-black overflow-y-hidden ohif-scrollbar">
       {thumbnails.map(
         ({
           displaySetInstanceUID,