fix(product): fix cart deduplication for products without variants

addToCart() compared item.variantId === selectedVariant where
selectedVariant is null but saved items store variantId: undefined.
null !== undefined caused findIndex to never match, always inserting
a duplicate instead of incrementing quantity. Normalised with || undefined.

app/src/app/products/[slug]/page.tsx

@@ -54,7 +54,7 @@ export default function ProductDetailPage() {
     const cart = JSON.parse(localStorage.getItem('cart') || '[]')
     const existingIndex = cart.findIndex(
       (item: { productId: string; variantId?: string }) =>
-        item.productId === product.id && item.variantId === selectedVariant
+        item.productId === product.id && item.variantId === (selectedVariant || undefined)
     )
 
     if (existingIndex >= 0) {
@@ -249,7 +249,12 @@ export default function ProductDetailPage() {
                 size="lg"
                 variant="secondary"
                 onClick={() => {
-                  addToCart()
+                  const cart = JSON.parse(localStorage.getItem('cart') || '[]')
+                  const alreadyInCart = cart.some(
+                    (item: { productId: string; variantId?: string }) =>
+                      item.productId === product.id && item.variantId === (selectedVariant || undefined)
+                  )
+                  if (!alreadyInCart) addToCart()
                   router.push('/cart')
                 }}
               >