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 <rotimi-best@users.noreply.github.com>

* 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 <rotimi-best@users.noreply.github.com>

---------

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

apps/dashboard/src/app.css

@@ -8,6 +8,28 @@
 
 @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-menu-elevated {
+  z-index: 250;
+}
+
 /* Geist variable font — self-hosted */
 @font-face {
   font-family: 'Geist';