refine admin cards and form headers

components/admin/media-field-picker.tsx

@@ -89,7 +89,7 @@ export function MediaFieldPicker({
   }, [serializedValue]);
 
   return (
-    <AppCard level={2} padding="sm" className="space-y-4">
+    <AppCard level={2} layer="single" padding="sm" className="space-y-4">
       <input ref={hiddenInputRef} type="hidden" name={inputName} value={serializedValue} />
 
       <div className="flex items-start justify-between gap-4">
@@ -130,7 +130,7 @@ export function MediaFieldPicker({
         </div>
       </div>
 
-      <AppCard padding="sm" className="rounded-nested border-border/70">
+      <AppCard layer="single" padding="sm" className="rounded-nested border-border/70">
         {selectedOption ? (
           <div className="flex items-center gap-4">
             <img