Preserve the Browser control secret boundary

[laotoutou]

Summary

• project Browser control responses from an explicit public-field allowlist
• keep runtime dashboard coordinates and proxy headers server-side
• assert the exact public response shape in the Browser service regression test

Context

The capability-aware Browser control path introduced in #62 accepted a runtime upstream object through structural typing. Spreading that object into the response could expose server-only upstream fields on the authenticated control endpoint. This patch returns only owner, transport, revision, and takeoverAvailable.

Validation

• npm test (749 tests)
• npm run build
• npm run openapi:check
• npm run typecheck
• npm run lint
• git diff --check