frontend: add dev proxy & CORS config for local cauweb <-> Python API

[ayushkrtiwari]

Goal

Make local dev experience smooth: cauweb dev server proxies API calls to Python backend to avoid CORS and auth friction.

Tasks

• Add proxy config (Vite/webpack devServer proxy) for /api/* -> http://localhost:8000.
• Add example .env.local with API_BASE, WS_URL, and explain env usage.
• Add CI check to ensure frontend doesn't use hardcoded backend URLs.

Acceptance criteria

• Running npm run dev with backend running allows cauweb to call APIs without CORS errors.