Update warning text and fix missing price in cart

- Change warning to say 'approximate placement' instead of just 'placement'
- Add missing cartItemId, name, mockup, unitPrice, and preview URLs when adding design to cart
- Fixes NaN price display for designs added from approval page

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

src/app/cart/page.tsx

@@ -166,7 +166,7 @@ export default function CartPage() {
           <p className="font-medium text-ink">Please check your design before checking out</p>
           <p className="mt-1 text-muted">
             We print exactly what&apos;s shown in your preview above — spelling, wording, and
-            placement included. Once your order&apos;s placed, that design goes straight to print,
+            approximate placement included. Once your order&apos;s placed, that design goes straight to print,
             so take a moment to make sure everything looks right.
           </p>
         </div>