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>

package.json

@@ -6,14 +6,13 @@
   "scripts": {
     "dev": "vite dev --host 0.0.0.0 --port 3000",
     "build": "vite build",
-    "typecheck": "tsc --noEmit && hydrogen gql check"
+    "typecheck": "tsc --noEmit"
   },
   "packageManager": "yarn@4.18.0+sha512.fcb8716fe7cd0eece141ffc18b92193a9df9204c1ba83189c288835223fc0bbe64af473bab0d5e9927a7daeb5caf2bb07eb2787cc9338ca040ea125f2a1f2f7e",
   "dependencies": {
     "@fontsource-variable/geist": "^5.3.0",
     "@fontsource-variable/geist-mono": "^5.3.0",
     "@remixicon/react": "^4.9.0",
-    "@shopify/hydrogen": "0.0.0-preview-116d5d7-20260730141607",
     "@tanstack/query-core": "^5.104.1",
     "@tanstack/react-query": "^5.104.1",
     "@tanstack/react-router": "^1.170.41",
@@ -23,7 +22,6 @@
     "clsx": "^2.1.1",
     "cmdk": "^1.1.1",
     "framer-motion": "^12.42.2",
-    "graphql": "^17.0.2",
     "hono": "^4.13.12",
     "lucide-react": "^0.562.0",
     "radix-ui": "^1.6.7",