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/Product.ts

@@ -0,0 +1,81 @@
+import { gql } from "@apollo/client";
+import { PRODUCT_VARIANT_FRAGMENT } from "./Variant";
+
+export const PRODUCT_FRAGMENT = gql`
+  fragment ProductFragment on Product {
+    id
+    name
+    slug
+    description
+    seoTitle
+    seoDescription
+    translation(languageCode: $locale) {
+      id
+      name
+      slug
+      description
+      seoTitle
+      seoDescription
+    }
+    variants {
+      ...ProductVariantFragment
+    }
+    media {
+      id
+      url
+      alt
+      type
+    }
+    category {
+      id
+      name
+      slug
+    }
+    metadata {
+      key
+      value
+    }
+  }
+  ${PRODUCT_VARIANT_FRAGMENT}
+`;
+
+export const PRODUCT_LIST_ITEM_FRAGMENT = gql`
+  fragment ProductListItemFragment on Product {
+    id
+    name
+    slug
+    description
+    translation(languageCode: $locale) {
+      id
+      name
+      slug
+      description
+    }
+    variants {
+      id
+      name
+      sku
+      quantityAvailable
+      pricing {
+        price {
+          gross {
+            amount
+            currency
+          }
+        }
+        onSale
+        discount {
+          gross {
+            amount
+            currency
+          }
+        }
+      }
+    }
+    media {
+      id
+      url
+      alt
+    }
+  }
+`;