Commit design approval workflow and related features

- Add design approval workflow components and API routes
- Update checkout process for design approval integration
- Add admin navigation for design management
- Update mail utilities for design notifications
- Update types for design approval system
- Update Prisma schema for design approval database

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

src/components/AdminNav.tsx

@@ -27,6 +27,9 @@ export default function AdminNav() {
       <Link href="/admin/inbox" className="tag-label hover:text-ink">
         Inbox
       </Link>
+      <Link href="/admin/designs" className="tag-label hover:text-ink">
+        Design Approvals
+      </Link>
     </div>
   );
 }