Update NavSidebar.scss to use z-index from _variables.scss

Previously, NavSidebar used a magic number for z-index. This commit changed this z-index to use  $z-index-above-base from _variables.scss.

Fixes #6633 where contextmenu--visible (part of ConversationPanel) is hidden behind NavSidebar.

Author: lvthanh03

stylesheets/components/NavSidebar.scss

@@ -3,7 +3,7 @@
 
 .NavSidebar {
   position: relative;
-  z-index: 10;
+  z-index: $z-index-base;
   height: 100%;
   display: flex;
   flex-direction: column;