fix(sidebar): scope elevated mobile z-index to course sidebar

Remove the global z-150 mobile sheet stacking from Sidebar.Root so LMS
and org sidebars keep the default sheet layer. Course sidebar opts in via
mobileOverlayClass and its existing z-150 class.

Co-authored-by: Rotimi Best <rotimi-best@users.noreply.github.com>

apps/dashboard/src/lib/features/course/components/sidebar/course-sidebar.svelte

@@ -112,7 +112,7 @@
 {#if !isOrgLoaded}
   <SidebarSkeleton />
 {:else}
-  <Sidebar.Root collapsible="icon" class="ui:z-150!">
+  <Sidebar.Root collapsible="icon" class="ui:z-150!" mobileOverlayClass="ui:z-150!">
     <Sidebar.Header>
       <CourseSidebarLogo />
     </Sidebar.Header>