Fix field name in DELETE endpoint - use designId not designApprovalId

The DesignApprovalMessage model uses designId as the foreign key,
not designApprovalId.

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

src/app/api/designs/[id]/route.ts

@@ -61,7 +61,7 @@ export async function DELETE(
   try {
     // Delete associated messages first
     await prisma.designApprovalMessage.deleteMany({
-      where: { designApprovalId: params.id },
+      where: { designId: params.id },
     });
 
     // Delete the design approval