feat: add privacy-safe WebUI history view

[laurieluo]

Summary

• add privacy-safe WebUI conversation history contracts, backend support, and tests
• update chat UI rendering and navigation presentation
• include ruff lint cleanup needed by the touched Python code
• bump the package/runtime version to 0.1.8

Validation

• uv run pytest tests/channels/test_webui_history.py tests/config/test_config_paths.py tests/privacy/webui
• npm test -- --run
• npm run build
• uv run python -c 'import cloakbot; print(cloakbot.version)'\n\nThis PR branch is based directly on upstream/main and contains the net changes as a single commit, so origin/main commit history is not carried into the upstream PR.