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/app/api/checkout/route.ts

@@ -19,6 +19,8 @@ type CheckoutCartItem = {
   designPreviewUrlBack: string | null;
   placementPreviewUrl: string | null;
   placementPreviewUrlBack: string | null;
+  designWidthCm: number | null;
+  designHeightCm: number | null;
 };
 
 type GuestInfo = {
@@ -111,6 +113,8 @@ export async function POST(req: Request) {
             designPreviewUrlBack: i.designPreviewUrlBack,
             placementPreviewUrl: i.placementPreviewUrl,
             placementPreviewUrlBack: i.placementPreviewUrlBack,
+            designWidthCm: i.designWidthCm,
+            designHeightCm: i.designHeightCm,
           })),
         },
       },