fix: enable scrolling in mobile navigation menu

The mobile category menu (``#navbarContentMobile``) could not scroll
when its content exceeded the viewport height, because no overflow
or height constraint was set on the collapse container.

Add ``max-height: calc(100vh - 60px)``, ``overflow-y: auto`` and
``-webkit-overflow-scrolling: touch`` to allow touch-scrolling
through the full category list on mobile devices.

resources/shop/css/site.css

@@ -397,6 +397,11 @@ div.megamenu ul.megamenu li.megamenu.level1
 }
 
 @media (max-width: 991.98px){
+	#navbarContentMobile {
+		max-height: calc(100vh - 60px);
+		overflow-y: auto;
+		-webkit-overflow-scrolling: touch;
+	}
 	#navbarContentMobile .navbar-nav {
 		flex-direction: column;
 	}