fix: Add z-index to sidebar menu container

Custom dashboards are rendered above the menu, causing visibility issue during menu transition

Author: pnrgenc

frontend/express/public/stylesheets/styles/blocks/_sidebar.scss

@@ -141,6 +141,7 @@ $bg-color: #24292e;
         box-sizing: border-box;
         height: 100%;
         overflow: hidden;
+        z-index: 2004;
         position: relative;
 
         /* .cly-vue-sidebar__menu--with-banner */