Fix cart page error when reading designJson

Handle designJson as either string or object, parse if needed.
Prevents TypeError when accessing front/back properties.

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

src/app/cart/page.tsx

@@ -12,7 +12,14 @@ export default function CartPage() {
   const removeItem = useCart((s) => s.removeItem);
   const setQuantity = useCart((s) => s.setQuantity);
   const total = useCart((s) => s.total());
-  const hasCustomDesign = items.some((item) => item.designJson.front.length > 0 || item.designJson.back.length > 0);
+  const hasCustomDesign = items.some((item) => {
+    try {
+      const design = typeof item.designJson === 'string' ? JSON.parse(item.designJson) : item.designJson;
+      return (design.front?.length ?? 0) > 0 || (design.back?.length ?? 0) > 0;
+    } catch {
+      return false;
+    }
+  });
 
   useEffect(() => setMounted(true), []);