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/fragments/Checkout.ts

@@ -0,0 +1,74 @@
+import { gql } from "@apollo/client";
+import { CHECKOUT_LINE_FRAGMENT } from "./Variant";
+
+export const ADDRESS_FRAGMENT = gql`
+  fragment AddressFragment on Address {
+    id
+    firstName
+    lastName
+    companyName
+    streetAddress1
+    streetAddress2
+    city
+    postalCode
+    country {
+      code
+      country
+    }
+    countryArea
+    phone
+    isDefaultBillingAddress
+    isDefaultShippingAddress
+  }
+`;
+
+export const CHECKOUT_FRAGMENT = gql`
+  fragment CheckoutFragment on Checkout {
+    id
+    token
+    email
+    isShippingRequired
+    lines {
+      ...CheckoutLineFragment
+    }
+    shippingPrice {
+      gross {
+        amount
+        currency
+      }
+    }
+    subtotalPrice {
+      gross {
+        amount
+        currency
+      }
+    }
+    totalPrice {
+      gross {
+        amount
+        currency
+      }
+    }
+    shippingAddress {
+      ...AddressFragment
+    }
+    billingAddress {
+      ...AddressFragment
+    }
+    shippingMethods {
+      id
+      name
+      price {
+        amount
+        currency
+      }
+    }
+    availablePaymentGateways {
+      id
+      name
+    }
+    note
+  }
+  ${CHECKOUT_LINE_FRAGMENT}
+  ${ADDRESS_FRAGMENT}
+`;