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.

components/admin/media-field-picker.tsx

@@ -89,7 +89,7 @@ export function MediaFieldPicker({
   }, [serializedValue]);
 
   return (
-    <AppCard level={2} layer="single" padding="sm" className="space-y-4">
+    <AppCard level={2} padding="sm" contentClassName="space-y-4">
       <input ref={hiddenInputRef} type="hidden" name={inputName} value={serializedValue} />
 
       <div className="flex items-start justify-between gap-4">