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]>

packages/ui/src/base/dialog/dialog-content.svelte

@@ -29,7 +29,7 @@
       // dialog's own bottom padding when a sticky footer is rendered —
       // the footer's own `py-4` provides the bottom spacing instead, and
       // it pins flush with the dialog's bottom edge.
-      'ui:bg-background ui:border ui:data-[state=open]:animate-in ui:data-[state=closed]:animate-out ui:data-[state=closed]:fade-out-0 ui:data-[state=open]:fade-in-0 ui:data-[state=closed]:zoom-out-95 ui:data-[state=open]:zoom-in-95 ui:fixed ui:left-[50%] ui:top-[50%] ui:z-200 ui:grid ui:w-full ui:max-w-[calc(100%-2rem)] ui:translate-x-[-50%] ui:translate-y-[-50%] ui:gap-4 ui:rounded-lg ui:p-6 ui:[&:has([data-slot=dialog-footer][data-sticky=true])]:pb-0 ui:shadow-lg ui:duration-200 ui:sm:max-w-lg',
+      'ui:bg-background ui:border ui:data-[state=open]:animate-in ui:data-[state=closed]:animate-out ui:data-[state=closed]:fade-out-0 ui:data-[state=open]:fade-in-0 ui:data-[state=closed]:zoom-out-95 ui:data-[state=open]:zoom-in-95 ui:fixed ui:left-[50%] ui:top-[50%] ui:z-modal ui:grid ui:w-full ui:max-w-[calc(100%-2rem)] ui:translate-x-[-50%] ui:translate-y-[-50%] ui:gap-4 ui:rounded-lg ui:p-6 ui:[&:has([data-slot=dialog-footer][data-sticky=true])]:pb-0 ui:shadow-lg ui:duration-200 ui:sm:max-w-lg',
       className
     )}
     {...restProps}

packages/ui/src/base/dialog/dialog-overlay.svelte

@@ -9,7 +9,7 @@
   bind:ref
   data-slot="dialog-overlay"
   class={cn(
-    'ui:data-[state=open]:animate-in ui:data-[state=closed]:animate-out ui:data-[state=closed]:fade-out-0 ui:data-[state=open]:fade-in-0 ui:fixed ui:inset-0 ui:z-200 ui:bg-black/50',
+    'ui:data-[state=open]:animate-in ui:data-[state=closed]:animate-out ui:data-[state=closed]:fade-out-0 ui:data-[state=open]:fade-in-0 ui:fixed ui:inset-0 ui:z-modal ui:bg-black/50',
     className
   )}
   {...restProps}

packages/ui/src/base/sidebar/sidebar.svelte

@@ -83,7 +83,7 @@
     <div
       data-slot="sidebar-container"
       class={cn(
-        'ui:w-(--sidebar-width) ui:fixed ui:inset-y-0 ui:z-100 ui:hidden ui:max-h-svh ui:transition-[left,right,width] ui:duration-200 ui:ease-linear ui:md:flex',
+        'ui:w-(--sidebar-width) ui:fixed ui:inset-y-0 ui:z-app-bar ui:hidden ui:max-h-svh ui:transition-[left,right,width] ui:duration-200 ui:ease-linear ui:md:flex',
         side === 'left'
           ? 'ui:left-0 ui:group-data-[collapsible=offcanvas]:left-[calc(var(--sidebar-width)*-1)]'
           : 'ui:right-0 ui:group-data-[collapsible=offcanvas]:right-[calc(var(--sidebar-width)*-1)]',