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/client.ts

@@ -0,0 +1,49 @@
+import { ApolloClient, InMemoryCache, createHttpLink } from "@apollo/client";
+import { setContext } from "@apollo/client/link/context";
+
+const httpLink = createHttpLink({
+  uri: process.env.NEXT_PUBLIC_SALEOR_API_URL || "http://localhost:8000/graphql/",
+});
+
+const authLink = setContext((_, { headers }) => {
+  // Saleor doesn't require auth for public queries
+  // Add auth token here if needed for admin operations
+  return {
+    headers: {
+      ...headers,
+      "Content-Type": "application/json",
+    },
+  };
+});
+
+export const saleorClient = new ApolloClient({
+  link: authLink.concat(httpLink),
+  cache: new InMemoryCache({
+    typePolicies: {
+      Query: {
+        fields: {
+          products: {
+            keyArgs: ["channel", "filter"],
+            merge(existing, incoming) {
+              if (!existing) return incoming;
+              return {
+                ...incoming,
+                edges: [...existing.edges, ...incoming.edges],
+              };
+            },
+          },
+        },
+      },
+    },
+  }),
+  defaultOptions: {
+    watchQuery: {
+      fetchPolicy: "cache-first",
+    },
+    query: {
+      fetchPolicy: "cache-first",
+    },
+  },
+});
+
+export default saleorClient;