Fix price display issues and layout in design review page

- Add defensive checks in cart store to handle missing/NaN unitPrice values
- Display '—' for items with invalid prices instead of NaN
- Fix design review page grid layout to align buttons at top of page
- Ensure buttons don't get pushed below images on review page

src/app/cart/page.tsx

@@ -147,7 +147,11 @@ export default function CartPage() {
             </div>
 
             <div className="w-24 text-right font-mono text-sm">
-              <Price cents={item.unitPrice * item.quantity} />
+              {Number.isFinite(item.unitPrice) ? (
+                <Price cents={item.unitPrice * item.quantity} />
+              ) : (
+                <span className="text-muted">—</span>
+              )}
             </div>
 
             <button