Migrate to TanStack Start and pin the Vercel preset

Vercel auto-detected the Hydrogen preset from the @shopify/hydrogen
dependency and failed building a 'hydrogen' Edge Function. The app is now
TanStack Start (SSR via Nitro), and vercel.json pins the tanstack-start
preset and installs with Corepack so Yarn 4 is used.

- Router factory with per-request QueryClient and SSR query dehydration
- Hono API mounted as a Start server route at /api/*
- Account reads in loaders go through server functions (cookies on SSR)
- Loaders prefetch listing pages so products are in the server HTML
- Add graphql peer dependency for gql.tada

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

src/hooks/use-account.ts

@@ -1,20 +1,15 @@
 import { queryOptions } from '@tanstack/react-query';
-import type { Customer } from '@/services/shopify/customer';
+import {
+  getAccountCustomer,
+  getSessionCustomer,
+} from '@/server/account.functions';
 
 // The customer token lives in an httpOnly cookie, so account state always comes
-// from the Hono API (server/account.ts) rather than from Shopify directly.
+// from the server (server functions here; mutations via src/server/account.ts).
 
-export interface SessionCustomer {
-  displayName: string;
-  email: string;
-  firstName?: string | null;
-}
-
-async function getJson<T>(url: string): Promise<T> {
-  const response = await fetch(url, { credentials: 'same-origin' });
-  if (!response.ok) throw new Error(`Request failed: ${response.status}`);
-  return response.json();
-}
+export type SessionCustomer = NonNullable<
+  Awaited<ReturnType<typeof getSessionCustomer>>
+>;
 
 /** Every account query shares this prefix, so sign-in/out can reset them all. */
 export const ACCOUNT_QUERY_KEY = ['account'] as const;
@@ -22,20 +17,14 @@ export const ACCOUNT_QUERY_KEY = ['account'] as const;
 export const sessionQueryOptions = () =>
   queryOptions({
     queryKey: [...ACCOUNT_QUERY_KEY, 'me'],
-    queryFn: () =>
-      getJson<{ customer: SessionCustomer | null }>('/api/account/me')
-        .then((data) => data.customer)
-        // A failed probe reads as signed out.
-        .catch(() => null),
+    // A failed probe reads as signed out.
+    queryFn: () => getSessionCustomer().catch(() => null),
   });
 
 export const customerQueryOptions = () =>
   queryOptions({
     queryKey: [...ACCOUNT_QUERY_KEY, 'customer'],
-    queryFn: () =>
-      getJson<{ customer: Customer | null }>('/api/account/customer').then(
-        (data) => data.customer
-      ),
+    queryFn: () => getAccountCustomer(),
     // Orders should reflect a purchase made a moment ago.
     staleTime: 0,
   });