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

package.json

@@ -5,7 +5,8 @@
     "scripts": {
         "dev": "next dev --webpack",
         "build": "next build --webpack",
-        "start": "next start"
+        "start": "next start",
+        "typecheck": "tsc --noEmit && hydrogen gql check"
     },
     "version": "0.0.0",
     "dependencies": {
@@ -15,6 +16,7 @@
         "@radix-ui/react-slot": "^1.3.3",
         "@radix-ui/react-use-controllable-state": "^1.2.6",
         "@remixicon/react": "^4.9.0",
+        "@shopify/hydrogen": "0.0.0-preview-116d5d7-20260730141607",
         "@shopify/storefront-api-client": "^1.0.0",
         "ai": "^7",
         "class-variance-authority": "0.7.1",
@@ -52,5 +54,6 @@
         "@types/react": "^19",
         "@types/react-dom": "^19",
         "typescript": "^5.7.3"
-    }
+    },
+    "packageManager": "yarn@4.18.0+sha512.fcb8716fe7cd0eece141ffc18b92193a9df9204c1ba83189c288835223fc0bbe64af473bab0d5e9927a7daeb5caf2bb07eb2787cc9338ca040ea125f2a1f2f7e"
 }