Replace Streamlit UI with FastAPI backend for computer-use demo

[ZenRepoMaster]

Summary

This PR replaces the experimental Streamlit interface with a production-capable
FastAPI backend, providing a cleaner architecture with proper session management,
real-time streaming, and persistent chat history.

What's changed

• FastAPI backend — REST API for session CRUD, SSE for real-time agent
  output, WebSocket proxy for VNC desktop access
• SQLite persistence — chat history and session state stored with
  SQLAlchemy async + aiosqlite; WAL mode for concurrent reads/writes
• API key encryption — Fernet symmetric encryption for keys stored at rest
• Multi-session support — per-session asyncio locks prevent concurrent
  sampling loop runs; SSE queue fan-out supports multiple browser tabs
• HTML/JS frontend — zero-dependency chat UI with three panels: session
  sidebar, chat messages (with inline screenshots), and a tabbed right panel
  for the VNC desktop and API request/response logs
• Docker Compose — two services: existing desktop (Xvfb + x11vnc + noVNC)
  and new api (FastAPI on port 8000); API waits for desktop healthcheck before
  starting

How to run

1. Copy .env.example to .env and fill in API_SECRET_KEY and
   ANTHROPIC_API_KEY
2. docker compose up
3. Open http://localhost:8000

What's preserved

• loop.py and all tools are unchanged
• Bedrock and Vertex provider support carried over
• noVNC iframe fallback still available on port 6080