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/app.css

@@ -8,32 +8,6 @@
 
 @custom-variant dark (&:where(.dark, .dark *));
 
-/**
- * Shared z-index scale, mirrored from `packages/ui/src/index.css`.
- *
- * Dashboard code has its own (unprefixed) Tailwind pipeline, separate from `@cio/ui`'s
- * `ui:`-prefixed one — a `ui:`-prefixed utility only compiles if that exact class string is
- * also used somewhere under `packages/ui/src`. So dashboard-only usages (page headers, the
- * course sidebar, dropdowns/selects that need to escape a sticky header or a modal) must use
- * these unprefixed names instead. Keep the two scales numerically in sync; see
- * `packages/ui/README.md` for the full write-up of each layer.
- */
-@utility z-app-bar {
-  z-index: 100;
-}
-
-@utility z-app-bar-elevated {
-  z-index: 150;
-}
-
-@utility z-modal {
-  z-index: 200;
-}
-
-@utility z-menu-elevated {
-  z-index: 250;
-}
-
 /* Geist variable font — self-hosted */
 @font-face {
   font-family: 'Geist';