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>

.gitignore

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