fix(dashboard): Certain menus inside of modals are hidden (#875)

* fix: update z-index utility class for Select component across tag category and tutor selection views

* refactor: standardize z-index management using ui:z-menu-elevated, ui:z-app-bar, ui:z-app-bar-elevated and ui:z-modal classes across UI components

---------

Co-authored-by: richardkenneth <[email protected]>

apps/dashboard/src/lib/features/course/pages/exercise.svelte

@@ -592,7 +592,7 @@
   }
 </script>
 
-<Page.Header isSticky={true} class="z-app-bar! top-12! min-h-[36px]">
+<Page.Header isSticky={true} class="ui:z-app-bar top-12! min-h-[36px]">
   <Page.HeaderContent>
     <Page.Title class="flex flex-col gap-2">
       <span>{exerciseDisplayTitle}</span>
@@ -638,7 +638,7 @@
                       </Button>
                     {/snippet}
                   </DropdownMenu.Trigger>
-                  <DropdownMenu.Content align="end" class="z-menu-elevated!">
+                  <DropdownMenu.Content align="end">
                     <DropdownMenu.Item onclick={addSectionFromHeader}>
                       {$t('course.navItem.lessons.exercises.all_exercises.add_section')}
                     </DropdownMenu.Item>
@@ -677,7 +677,7 @@
                       </Button>
                     {/snippet}
                   </DropdownMenu.Trigger>
-                  <DropdownMenu.Content align="end" class="z-menu-elevated!">
+                  <DropdownMenu.Content align="end">
                     {#if hasSections($questionnaire.sections)}
                       <DropdownMenu.Item onclick={() => (reorderQuestions = !reorderQuestions)}>
                         {reorderQuestions