Fix: Handle designJson as string in checkout API

The designJson field can be either a parsed object or a JSON string
depending on how it was stored. The checkout API was assuming it was
always an object and casting it, which caused 'Cannot read properties
of undefined' error when trying to access .length on front/back arrays.

Now properly parses the string if needed and safely accesses array
properties with optional chaining.

src/app/api/checkout/route.ts

@@ -78,8 +78,8 @@ export async function POST(req: Request) {
 
     const activePromotions = await fetchActivePromotions();
     const pricedItems = items.map((i) => {
-      const design = i.designJson as { front: unknown[]; back: unknown[] };
-      const isPersonalised = design.front.length > 0 || design.back.length > 0;
+      const design = typeof i.designJson === 'string' ? JSON.parse(i.designJson) : i.designJson;
+      const isPersonalised = (design.front?.length ?? 0) > 0 || (design.back?.length ?? 0) > 0;
       return {
         ...i,
         unitPrice: getEffectivePrice(productsById.get(i.productId)!, activePromotions, new Date(), isPersonalised).price,