feat(saleor): Phase 1 - GraphQL Client Setup

- Add Apollo Client for Saleor GraphQL API
- Create GraphQL fragments (Product, Variant, Checkout)
- Create GraphQL queries (Products, Checkout)
- Create GraphQL mutations (Checkout operations)
- Add TypeScript types for Saleor entities
- Add product helper functions
- Install @apollo/client and graphql dependencies

Part of WordPress/WooCommerce → Saleor migration

src/lib/saleor/queries/Checkout.ts

@@ -0,0 +1,21 @@
+import { gql } from "@apollo/client";
+import { CHECKOUT_FRAGMENT } from "../fragments/Checkout";
+
+export const GET_CHECKOUT = gql`
+  query GetCheckout($token: UUID!) {
+    checkout(token: $token) {
+      ...CheckoutFragment
+    }
+  }
+  ${CHECKOUT_FRAGMENT}
+`;
+
+export const GET_CHECKOUT_BY_ID = gql`
+  query GetCheckoutById($id: ID!) {
+    checkout(id: $id) {
+      ...CheckoutFragment
+    }
+  }
+  ${CHECKOUT_FRAGMENT}
+`;
+`;