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/collections.ts

@@ -0,0 +1,91 @@
+import { gql } from '@shopify/hydrogen';
+import { ProductFragment } from '@/graphql/products';
+
+// Get all collections
+export const GET_COLLECTIONS_QUERY = gql(`
+  query GetCollections($first: Int!, $country: CountryCode, $language: LanguageCode)
+  @inContext(country: $country, language: $language) {
+    collections(first: $first) {
+      edges {
+        node {
+          id
+          title
+          handle
+          description
+          descriptionHtml
+          image {
+            id
+            url
+            altText
+            width
+            height
+          }
+        }
+      }
+      pageInfo {
+        hasNextPage
+        endCursor
+      }
+    }
+  }
+`);
+
+// Get products in a collection
+export const GET_COLLECTION_PRODUCTS_QUERY = gql(
+  `
+  query GetCollectionProducts(
+    $handle: String!
+    $first: Int!
+    $after: String
+    $sortKey: ProductCollectionSortKeys
+    $reverse: Boolean
+    $filters: [ProductFilter!]
+    $country: CountryCode
+    $language: LanguageCode
+  ) @inContext(country: $country, language: $language) {
+    collection(handle: $handle) {
+      id
+      title
+      handle
+      description
+      descriptionHtml
+      image {
+        id
+        url
+        altText
+        width
+        height
+      }
+      products(
+        first: $first
+        after: $after
+        sortKey: $sortKey
+        reverse: $reverse
+        filters: $filters
+      ) {
+        filters {
+          id
+          label
+          type
+          values {
+            id
+            label
+            count
+            input
+          }
+        }
+        edges {
+          node {
+            ...ProductFragment
+          }
+        }
+        pageInfo {
+          hasNextPage
+          endCursor
+        }
+      }
+    }
+  }
+`,
+  [ProductFragment]
+);