[codex] Align React frontend with PRD runtime basics

[2002yy]

Summary

• Add the PRD baseline document for the Streamlit-to-React feature migration.
• Add backend runtime, role, and memory status APIs for the new React frontend.
• Wire React settings, RAG options, role preview, route inspection, sessions, and memory status panels to real API data.

Validation

• .\.venv\Scripts\python.exe -m pytest tests/test_api.py -q -> 22 passed
• npm.cmd run build -> passed

Notes

This is the first P0 migration slice. Group chat and network/news search are still pending follow-up slices.