#183 feat(demo): add connection preflight

Summary

Adds a small preflight status section to the local demo so first-time users can immediately see whether the main local layers are available:

• demo server
• Browser Harness daemon
• Chrome / CDP connection

When Browser Harness or Chrome/CDP is unavailable, the UI points users to:

uv run browser-harness --doctor

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## Summary by cubic
Adds a connection status section to the local demo so first-time users can immediately see whether the demo server, Browser Harness daemon, and Chrome/CDP are available.

- Adds a `/api/health` endpoint that checks the daemon and Chrome via CDP and logs probe failures to the server console.
- Shows each layer as Ready or Unavailable and suggests `uv run browser-harness --doctor` when a layer is missing.
- Adds tests covering healthy, missing-harness, and CDP-failure states.

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