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>

.gitignore

@@ -1,6 +1,9 @@
 .DS_Store
 node_modules/*
 dist
+.output
+.nitro
+.vercel
 .yarn/install-state.gz
 .env*.local
 src/routeTree.gen.ts