refactor(z-index): introduce a shared named z-index scale (#723)

* refactor(z-index): introduce a shared named z-index scale

Replace scattered magic z-index numbers (100, 150, 200, 201, 250) with
named layers (z-app-bar, z-app-bar-elevated, z-modal, z-menu-elevated)
so intent is documented at each call site and future additions insert
predictably instead of colliding.

Defined via Tailwind's @utility directive in two places that mirror
each other: packages/ui/src/index.css (ui:-prefixed, for components
that live in the @cio/ui package) and apps/dashboard/src/app.css
(unprefixed, for dashboard-only files) — a ui:-prefixed custom utility
only compiles if it is also used somewhere under packages/ui/src, so
dashboard-only usages need the unprefixed twin.

Also fixes two pre-existing dropdown/select menus (tags-admin.svelte,
mark-exercise-modal.svelte) that used ui:z-[250], which never actually
compiled to any CSS rule since nothing under packages/ui/src used that
exact class.

Co-authored-by: Rotimi Best <[email protected]>

* fix(ui): safelist the packaged z-index scale so it always compiles

@utility only emits a class when Tailwind's scanner sees that exact
candidate under packages/ui/src, so ui:z-app-bar-elevated and
ui:z-menu-elevated were documented but silently missing from the
compiled @cio/ui CSS since nothing inside the package itself used
them. Safelist all four layers via @source inline() so the shipped
scale always matches what's documented, regardless of which layers
this package happens to reference internally.

Co-authored-by: Rotimi Best <[email protected]>

---------

Co-authored-by: Cursor Agent <[email protected]>
Co-authored-by: Rotimi Best <[email protected]>

apps/dashboard/src/lib/features/course/components/exercise/mark-exercise-modal.svelte

@@ -296,7 +296,7 @@
               >
                 <EllipsisVerticalIcon class="h-5 w-5" />
               </DropdownMenu.Trigger>
-              <DropdownMenu.Content align="end" class="ui:z-[250]">
+              <DropdownMenu.Content align="end" class="z-menu-elevated">
                 <DropdownMenu.Item
                   class="text-red-600"
                   onclick={() => {
@@ -366,7 +366,7 @@
               <Select.Trigger class="w-full">
                 {status.text}
               </Select.Trigger>
-              <Select.Content class="ui:z-[250]">
+              <Select.Content class="z-menu-elevated">
                 {#each SELECTABLE_STATUS as statusOption}
                   <Select.Item value={statusOption.id.toString()}>
                     {statusOption.text}