Scaffold Start and port the routes

The structural move: same routes, new framework shell.

• Set up TanStack Start on Vite, port the file-based routes across (they're already TanStack Router files, so this is mostly mechanical)
• The frontend container becomes a node server instead of nginx serving static files: new Dockerfile, healthcheck, Traefik keeps routing the same host to it
• Loaders keep using React Query + the RPC client
• Everything works exactly as before, SSR itself comes in the next ticket