Move Storefront queries onto @shopify/hydrogen preview client

Replaces the hand-rolled shopifyFetch wrapper and raw query strings with
the hydrogen preview package (0.0.0-preview-116d5d7-20260730141607), the
same build pinned in hydrogen-preview/.

- services/shopify/client: createStorefrontClient({type: "public"}) with a
  static request context. Two module-scoped clients — uncached for carts,
  products and customers; revalidate-3600 for shop policies — using the
  client's custom fetch option to carry Next's caching hints.
- graphql/*: every document wrapped in gql(), fragments composed via the
  second argument instead of string interpolation, and $country/$language
  declared with @inContext so hydrogen injects them.
- shopifyFetch keeps its {query, variables} call shape so call sites are
  unchanged, but is now generic over the document, so data is inferred. It
  re-raises GraphQL errors, which hydrogen returns rather than throws.

Env var names, the 2025-07 API version pin, and the client-side fetching
architecture are unchanged.

Turning on type coverage surfaced real defects, not just annotations:

- hydrogen gql check caught $discountCodes: [String!] used where the field
  requires [String!]!.
- Cart and customer mutation payloads are nullable and were dereferenced
  unconditionally, so a failed mutation threw a TypeError. Adds a shared
  unwrapCartPayload helper (collapsing five copies of the same userErrors
  check) and explicit null handling in the customer service, so a null
  payload reads as an error rather than success with no errors.
- Search results are a Product | Page | Article union; adds __typename to
  the queries and narrows on it.
- Widens nullable fields (altText, image, customer.email, totalTaxAmount)
  in the domain interfaces and in the structural duplicates some
  components declare locally.

Adds a typecheck script (tsc --noEmit && hydrogen gql check); it passes
with 0 errors. Two deprecation warnings are left alone as acting on them
would change behaviour: ProductOption.values and CartCost.totalTaxAmount.

Verified against mock.shop: build prerenders the policy pages through the
cached client, and in the browser the product grid, search with facets,
collection filter round-trip, add-to-cart and quantity update all work.
Customer account flows are untested — they need real credentials.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JaxgqPbFxLsSuLPZom2kdC

graphql/products.ts

@@ -0,0 +1,150 @@
+import { gql } from '@shopify/hydrogen';
+
+// Product Fragment for consistent product data
+export const ProductFragment = gql(`
+  fragment ProductFragment on Product {
+    id
+    title
+    handle
+    description
+    descriptionHtml
+    vendor
+    productType
+    tags
+    availableForSale
+    priceRange {
+      minVariantPrice {
+        amount
+        currencyCode
+      }
+      maxVariantPrice {
+        amount
+        currencyCode
+      }
+    }
+    compareAtPriceRange {
+      minVariantPrice {
+        amount
+        currencyCode
+      }
+      maxVariantPrice {
+        amount
+        currencyCode
+      }
+    }
+    images(first: 10) {
+      edges {
+        node {
+          id
+          url
+          altText
+          width
+          height
+        }
+      }
+    }
+    variants(first: 100) {
+      edges {
+        node {
+          id
+          title
+          availableForSale
+          quantityAvailable
+          selectedOptions {
+            name
+            value
+          }
+          price {
+            amount
+            currencyCode
+          }
+          compareAtPrice {
+            amount
+            currencyCode
+          }
+          image {
+            id
+            url
+            altText
+            width
+            height
+          }
+        }
+      }
+    }
+    options {
+      id
+      name
+      values
+      optionValues {
+        id
+        name
+        swatch {
+          color
+          image {
+            previewImage {
+              url
+            }
+          }
+        }
+        firstSelectableVariant {
+          id
+          image {
+            id
+            url
+            altText
+            width
+            height
+          }
+        }
+      }
+    }
+  }
+`);
+
+// Get multiple products
+export const GET_PRODUCTS_QUERY = gql(
+  `
+  query GetProducts($first: Int!, $after: String, $query: String, $sortKey: ProductSortKeys, $reverse: Boolean, $country: CountryCode, $language: LanguageCode)
+  @inContext(country: $country, language: $language) {
+    products(first: $first, after: $after, query: $query, sortKey: $sortKey, reverse: $reverse) {
+      edges {
+        node {
+          ...ProductFragment
+        }
+      }
+      pageInfo {
+        hasNextPage
+        endCursor
+      }
+    }
+  }
+`,
+  [ProductFragment]
+);
+
+// Get a single product by handle
+export const GET_PRODUCT_QUERY = gql(
+  `
+  query GetProduct($handle: String!, $country: CountryCode, $language: LanguageCode)
+  @inContext(country: $country, language: $language) {
+    product(handle: $handle) {
+      ...ProductFragment
+    }
+  }
+`,
+  [ProductFragment]
+);
+
+// Get product recommendations
+export const QUERY_PRODUCT_RECOMMENDATIONS = gql(
+  `
+  query GetProductRecommendations($productId: ID!, $country: CountryCode, $language: LanguageCode)
+  @inContext(country: $country, language: $language) {
+    productRecommendations(productId: $productId) {
+      ...ProductFragment
+    }
+  }
+`,
+  [ProductFragment]
+);