Deploy to Vercel as a TanStack Start app via Nitro

Vercel served 404s: with Hydrogen gone it treated the app as a static
Vite site, but TanStack Start's build has no index.html; pages come from
its server. Add the hosting adapter TanStack documents for Vercel:

- nitro() in vite.config, for 'vite build' only (dev and the nodesandbox
  runtime never load it); on Vercel it writes .vercel/output with the
  static assets and one Node.js function running Start + Hono
- vercel.json: tanstack-start preset, Yarn 4 via Corepack
- 'yarn start' runs the Node build; canonical URLs use the Vercel domain

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

vite.config.mjs

@@ -5,14 +5,24 @@ import tailwindcss from '@tailwindcss/vite';
 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; set VITE_SITE_URL in production.
-const siteUrl = process.env.VITE_SITE_URL ?? 'http://localhost:3000';
+// the server render and the client agree on it. On Vercel the production domain
+// is used automatically; VITE_SITE_URL overrides 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({
+export default defineConfig(async ({ command }) => ({
   server: { port: 3000 },
   plugins: [
     // Start must come before the React plugin.
     tanstackStart(),
+    // Hosting adapter for `vite build`: Nitro packages Start's server for the
+    // host (a Vercel Function when building on Vercel, a Node server in
+    // .output/ elsewhere). Dev never loads it, so `vite dev` — and the
+    // nodesandbox runtime — run plain Vite + TanStack Start + Hono.
+    ...(command === 'build' ? [(await import('nitro/vite')).nitro()] : []),
     react(),
     tailwindcss(),
   ],
@@ -24,4 +34,4 @@ export default defineConfig({
       '@': fileURLToPath(new URL('./src', import.meta.url)),
     },
   },
-});
+}));