frontend: synthetic demo mode + dev-only WS generator for cauweb

[ayushkrtiwari]

Goal

Allow contributors to run cauweb locally with a synthetic data generator to demo real-time flows without backend.

Tasks

• Add scripts/mock-ws-server.js (node) that emits price ticks, order updates, and heartbeats with adjustable seed/rate.
• Add ?mock=true dev mode in frontend to auto-connect to mock server.
• Add docs: how to run mock server and demo the UI (one-liners).
• Add Playwright tests that can toggle mock mode.

Acceptance criteria

• Running mock server + npm run dev shows live updating UI without backend.