- Add runs board screenshot to introduction page
- Add run detail and workflow diagram screenshots to why-arc page
- Add private research preview warning to quick-start page
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Replace the server-level `--demo` flag with per-request demo dispatch.
The Rust API builds both a demo and real router; incoming requests with
the `X-Arc-Demo: 1` header hit the demo router (auth disabled, static
data), all others hit the real router with normal auth.
The React web app gets a beaker icon toggle in the top nav bar (next to
the theme toggle) that sets an `arc-demo` cookie. Loaders read the
cookie to decide whether to send the `X-Arc-Demo: 1` header to the API.
The `ARC_DEMO=1` env var still works as a default when no cookie is set.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
