Call hydrogen's storefront.graphql directly, drop the compat shim

Completes the migration started in f8c3ef9. That commit put hydrogen
underneath the existing shopifyFetch wrapper; this one removes the
wrapper so every query uses the package's own client API.

- All 22 call sites now call storefront.graphql(DOCUMENT, { variables })
  (cachedStorefront for shop policies) instead of shopifyFetch, which
  emulated the old client's {query, variables} -> {data} shape.
- shopifyFetch is replaced by unwrapStorefrontResult(result, operation),
  which is only an error policy, not a transport wrapper: it returns
  data and throws when Shopify reports GraphQL errors, since hydrogen
  returns those rather than throwing. Naming the operation means a
  failure points at the call site instead of just at "Shopify".
- Environment reads move to services/shopify/config, so client.ts is
  only the hydrogen clients and shop-pay-button no longer imports from
  a query module just to get the store domain.
- Removes @shopify/storefront-api-client, the superseded client. It had
  no importers.

The SHOPIFY_STOREFRONT_API_URL export is gone too — hydrogen builds the
endpoint from storeDomain and apiVersion, and nothing else used it.

No behaviour change intended: same documents, same env var names, same
2025-07 API pin, same caching split between the two clients.

yarn typecheck passes with 0 errors. Verified against mock.shop: build
prerenders the policy pages, and in the browser product grid, search,
collection filters, cart restore, quantity update and discount-code
apply all work with no console errors. Customer account flows remain
untested — they need real credentials.

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

components/shopify/shop-pay-button.tsx

@@ -1,7 +1,7 @@
 'use client';
 
 import React from 'react';
-import { SHOPIFY_STORE_DOMAIN } from '@/services/shopify/client';
+import { SHOPIFY_STORE_DOMAIN } from '@/services/shopify/config';
 import ShopPayLogo from '@/components/shopify/shop-pay-logo';
 import { Loader } from '@/components/ui/loader';
 import { Button } from '@/components/ui/button';