frontend: end-to-end tests for cauweb with mocked WS server (Playwright)

[ayushkrtiwari]

Goal

Add reliable e2e tests that verify WS-driven UI updates and critical user flows.

Tasks

• Add Playwright test harness that launches the app and a mock WS server.
• Test scenarios:
  • connect + receive tick updates -> chart shows data
  • Start backtest -> job progress updates via WS
  • Place order -> optimistic UI -> server confirm
• Add CI job to run e2e tests on PRs labelled e2e or nightly.

Acceptance criteria

• Playwright tests pass locally and in CI with mock WS.