Fixes #8931: stream onboarding activity in dashboard

[T-rav]

Summary

• add NDJSON stream endpoints for onboarding materialize and push activity
• wire BootstrapWizard and ProjectView to stream activity into their logs with JSON fallback support
• add dashboard/context/backend coverage, including a no-terminal Acme/claims-ledger third-domain wizard flow

Verification

• uv run pytest tests/test_onboarding_api.py -q
• npm test -- --run src/components/tests/BootstrapWizard.test.jsx src/components/tests/ProjectView.test.jsx src/context/tests/HydraFlowContext.test.jsx
• make quality

Fixes #8931