frontend: connect cauweb to backtest REST + WS endpoints (real-time run/status updates)

[ayushkrtiwari]

Goal

Wire cauweb UI to backend endpoints so users can launch backtests and watch live status/progress via WS.

Tasks

• Add UI flow: Start backtest (POST /api/backtest), show job id and live progress.
• Listen to WS topic backtest:{jobId} to receive incremental metrics (progress, partial PnL, errors).
• Show live progress UI (progress bar, latest metrics, live chart updates).
• Add error & retry UX: if job fails, display logs and offer re-run.
• Add tests mocking REST + WS flows.

Acceptance criteria

• Starting a backtest shows a live-updating progress panel and final results without full page reload.