Fix undefined customerName variable in design submission

Changed customerName to customer.name to use the actual customer object.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/app/api/designs/submit/route.ts

@@ -63,7 +63,7 @@ export async function POST(req: Request) {
 
     const adminLink = `${process.env.NEXT_PUBLIC_SITE_URL || 'http://localhost:3000'}/admin/designs/${design.id}`;
     await sendDesignSubmissionNotification({
-      customerName,
+      customerName: customer.name,
       productName: product.name,
       adminLink,
     });