Epic 9: Move the frontend to TanStack Start

Move the frontend from a Vite SPA to TanStack Start so the public pages render on the server. Right now a shared review link unfurls as a blank card because there's no HTML and no meta tags until JS runs, and crawlers see an empty page. The router, routes and query patterns carry over since it's all TanStack already. Admin pages stay client-rendered behind the existing auth. Lock the Start version until it's fully stable.