STYLED - Roll out the layered card system to the admin workspace

The double-layer AppCard look showcased in the UI Kit was only applied on
the public frontend; every admin surface was pinned to layer=single and
stayed flat. Convert the admin section/container cards to the default
layered look, keeping compact metric tiles, toolbars, image tiles and
nested leaf cards single.

Add AppCard contentClassName so content utilities (space-y-*, grid, flex)
keep applying to the inner shell after the switch to double layer.

app/_admin/maintenance/page.tsx

@@ -60,7 +60,7 @@ export default async function AdminMaintenancePage({
       headerDescription={copy.subtitle}
     >
       <MotionFade delay={0.1}>
-        <AppCard layer="single">
+        <AppCard>
           <CardContent className="space-y-4 p-6">
             <p className="text-sm text-muted-foreground">{copy.maintenanceText}</p>
             <div className="flex flex-wrap items-center gap-3">