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>

vite.config.ts

@@ -1,30 +1,36 @@
 import { defineConfig } from 'vite';
+import { tanstackStart } from '@tanstack/react-start/plugin/vite';
+import { nitro } from 'nitro/vite';
 import react from '@vitejs/plugin-react';
 import tailwindcss from '@tailwindcss/vite';
-import { tanstackRouter } from '@tanstack/router-plugin/vite';
-import devServer from '@hono/vite-dev-server';
 import { fileURLToPath, URL } from 'node:url';
 
+// Absolute origin for canonical and Open Graph URLs. Inlined at build time so
+// the server render and the client agree on it. Vercel provides the production
+// domain; set VITE_SITE_URL to override it anywhere.
+const siteUrl =
+  process.env.VITE_SITE_URL ??
+  (process.env.VERCEL_PROJECT_PRODUCTION_URL
+    ? `https://${process.env.VERCEL_PROJECT_PRODUCTION_URL}`
+    : 'http://localhost:3000');
+
 export default defineConfig({
+  server: { port: 3000 },
   plugins: [
-    // Must run before the React plugin so the generated route tree is in place.
-    tanstackRouter({ target: 'react', autoCodeSplitting: true }),
+    // Start must come before the React plugin.
+    tanstackStart(),
+    // Builds the server for the host: Vercel's Build Output when deployed
+    // there, a standalone Node server (.output/) everywhere else.
+    nitro(),
     react(),
     tailwindcss(),
-    // Serves the Hono API in dev. Everything outside /api falls through to
-    // Vite, which serves the SPA.
-    devServer({
-      entry: 'server/app.ts',
-      exclude: [/^(?!\/api(\/|$)).*/],
-      injectClientScript: false,
-    }),
   ],
+  define: {
+    'import.meta.env.VITE_SITE_URL': JSON.stringify(siteUrl),
+  },
   resolve: {
     alias: {
       '@': fileURLToPath(new URL('./src', import.meta.url)),
     },
   },
-  build: {
-    outDir: 'dist/client',
-  },
 });