frontend: implement typed WebSocket client wrapper (TS) with topic subscriptions

[ayushkrtiwari]

Goal

Create a robust TypeScript WebSocket client used by cauweb for realtime updates (topics, reconnect, schema validation).

Tasks

• Add frontend/src/lib/wsClient.ts (TS) implementing:
  • typed message interfaces (Request/Response/Event)
  • subscribe(topic)/unsubscribe(topic) semantics
  • exponential reconnect with jitter and backoff
  • heartbeat/ping-pong handling and stale detection
• Use JSON Schema or zod for runtime validation of inbound messages.
• Add a simple mock server used by unit tests and Playwright e2e.
• Add docs: usage examples and error handling patterns.

Acceptance criteria

• Components can call useWS.subscribe('price:AAPL') and receive typed messages.
• Client reconnects and resumes subscriptions automatically.