Feature: Add quantity adjustment and fix shipping selection

- Add quantity adjustment controls (+ and -) on checkout order summary
- Prevent quantity from going below 1 with disabled minus button at qty=1
- Fix shipping selector not updating by wrapping callback in useCallback
- Shipping option changes now properly update order total
- Totals recalculate reactively when adjusting quantities or shipping

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

src/components/StandardProductView.tsx

@@ -5,6 +5,7 @@ import { v4 as uuid } from 'uuid';
 import ProductMockup from './ProductMockup';
 import Price from './Price';
 import LoginPromptModal from './LoginPromptModal';
+import AddToCartModal from './AddToCartModal';
 import { useCart } from '@/lib/cartStore';
 import { getColorName } from '@/lib/colorNames';
 import { checkCustomerAuth } from '@/app/actions';
@@ -17,6 +18,7 @@ export default function StandardProductView({ product }: { product: ProductDTO }
   const [quantity, setQuantity] = useState(1);
   const [justAdded, setJustAdded] = useState(false);
   const [showLoginPrompt, setShowLoginPrompt] = useState(false);
+  const [showAddToCartModal, setShowAddToCartModal] = useState(false);
   const addItem = useCart((s) => s.addItem);
 
   const hasBack = Boolean(product.imageUrlBack);
@@ -48,11 +50,6 @@ export default function StandardProductView({ product }: { product: ProductDTO }
   }
 
   const handleAddToBag = async () => {
-    const { isAuthenticated } = await checkCustomerAuth();
-    if (!isAuthenticated) {
-      setShowLoginPrompt(true);
-    }
-
     addItem({
       cartItemId: uuid(),
       productId: product.id,
@@ -69,8 +66,7 @@ export default function StandardProductView({ product }: { product: ProductDTO }
       placementPreviewUrl: product.colorPhotos[color] ?? product.imageUrl ?? null,
       placementPreviewUrlBack: product.imageUrlBack ?? null,
     });
-    setJustAdded(true);
-    setTimeout(() => setJustAdded(false), 2200);
+    setShowAddToCartModal(true);
   };
 
   return (
@@ -188,10 +184,14 @@ export default function StandardProductView({ product }: { product: ProductDTO }
             Add to bag — <Price cents={product.effectivePrice * quantity} />
           </button>
         </div>
-        {justAdded && <p className="text-sm text-pine">Added to your bag.</p>}
       </div>
 
       <LoginPromptModal isOpen={showLoginPrompt} onClose={() => setShowLoginPrompt(false)} />
+      <AddToCartModal
+        isOpen={showAddToCartModal}
+        onClose={() => setShowAddToCartModal(false)}
+        productName={product.name}
+      />
     </div>
   );
 }