Route /api/auth through the same origin in both dev and prod so
cookies stay first-party and the browser and React Router SSR share
one auth surface.
- Vite dev server proxies /api/auth to the Convex HTTP site.
- Better Auth uses secure cookies when the site URL is https.
- Add docs/auth-proxy.md documenting the required production ingress
(Caddy/Traefik) and Convex SITE_URL / CONVEX_SITE_URL setup.
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