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/Variant.ts

@@ -0,0 +1,84 @@
+import { gql } from "@apollo/client";
+
+export const PRODUCT_VARIANT_FRAGMENT = gql`
+  fragment ProductVariantFragment on ProductVariant {
+    id
+    name
+    sku
+    quantityAvailable
+    weight {
+      value
+      unit
+    }
+    media {
+      id
+      url
+      alt
+    }
+    pricing {
+      price {
+        gross {
+          amount
+          currency
+        }
+        net {
+          amount
+          currency
+        }
+      }
+      onSale
+      discount {
+        gross {
+          amount
+          currency
+        }
+      }
+    }
+    attributes {
+      attribute {
+        name
+        slug
+      }
+      values {
+        name
+        slug
+      }
+    }
+  }
+`;
+
+export const CHECKOUT_LINE_FRAGMENT = gql`
+  fragment CheckoutLineFragment on CheckoutLine {
+    id
+    quantity
+    totalPrice {
+      gross {
+        amount
+        currency
+      }
+    }
+    variant {
+      id
+      name
+      sku
+      product {
+        id
+        name
+        slug
+        media {
+          id
+          url
+          alt
+        }
+      }
+      pricing {
+        price {
+          gross {
+            amount
+            currency
+          }
+        }
+      }
+    }
+  }
+`;