feat: reliable browser challenge handoff and optional local Chrome

[davidmckayv]

Browser challenges could strand a Bot or let it continue with stale page state. This change detects Cloudflare Challenge Pages and conservative visible challenge signals, pauses computer actions, and lets a person take over the existing page. Handback resumes the exact request after a fresh snapshot; cancellation, expiry, and browser interruption remain distinct outcomes.

The viewer reconnects without releasing human control, and a reloaded conversation offers an explicit Resume task action for its pending browser tool. Managed browsing now uses full Chromium. Local API deployments can opt into installed Chrome with separate per-Bot profiles, a loopback endpoint, the browser sandbox retained, and shell execution disabled. Setup is documented in docs/configuration.md.

Validation:

• Repository CI suite: 5,290 passed, 36 existing opt-in skips, zero failures; affected follow-up regressions passed.
• App, server, and computer typechecks; scoped formatting/lint; app production build and server build passed.
• Actual full headless Chromium navigated, read, and snapshotted example.com. Actual installed Chrome rendered in the live viewer.
• The selected General Assistant rendered a real chart and used its browser tool. A synthetic challenge completed through detection, takeover, human input, handback, and a fresh browser read of the success phrase.
• Reloaded the app during a waiting challenge: the exact request and browser page survived, Resume task continued the original tool call, and the Bot read the completed page successfully. Fresh-snapshot enforcement remained set after handback until Resume task captured the new state.

Detection is deliberately conservative. Some embedded or score-only challenges are not automatically identifiable; model-requested handoff remains available. These checks establish the browser and handoff behavior, not guaranteed acceptance by Cloudflare, Google, or any other website.