Add React frontend for energy trading system

Implements React + TypeScript UI with Vite and Tailwind CSS.
Features dashboard with real-time WebSocket updates, backtesting
page, model management interface, trading controls, and settings.
Includes state management with Zustand, API integration with
Axios/TanStack Query, and interactive charts with Recharts.

frontend/.env.example

@@ -0,0 +1,2 @@
+VITE_API_URL=http://localhost:8000
+VITE_WS_URL=ws://localhost:8000/ws/real-time