Replace @shopify/hydrogen with direct Storefront API queries

Vercel's 'Hydrogen (v1)' preset matches any project depending on
@shopify/hydrogen and runs its v1 builder, which fails looking for
dist/worker. We only used the preview package as a Storefront API client,
so talk to the API directly instead:

- services/shopify/client.ts: plain fetch to /api/{version}/graphql.json
  with the public token header, @inContext country/language defaults, and
  the same graphql() / unwrapStorefrontResult API as before
- graphql/gql.ts: small helper that joins a document with its fragments
  (de-duplicated); the queries themselves are unchanged
- Explicit response types at each call site in place of the generated
  gql.tada types; local ProductFilter input type
- Drop @shopify/hydrogen, graphql, the TS plugin and 'hydrogen gql check'
- Copy no longer mentions Hydrogen

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tsconfig.json

@@ -12,7 +12,6 @@
     "isolatedModules": true,
     "jsx": "react-jsx",
     "types": ["vite/client", "node"],
-    "plugins": [{ "name": "@shopify/hydrogen/ts-plugin" }],
     "paths": {
       "@/*": ["./src/*"]
     }