[Testing] Add Selenium and API tests for core BookRunner flows

[Josan88]

Summary

Add automated verification so the project can demonstrate repeatable quality checks before deployment.

Tasks

• Set up backend API testing with Jest and Supertest
• Cover health, registration/login, cart, checkout/order creation, and purchase history endpoints
• Set up Selenium frontend tests for key user flows
• Cover registration or login, product browsing, cart operations, checkout, and purchase history navigation
• Add test data setup and cleanup where needed
• Make test commands easy to run locally and in CI

Done criteria

• Documented test commands pass locally
• Backend health and core user journey are covered by API tests
• Key frontend flows are covered by Selenium tests
• Test failures are visible and usable in GitHub Actions

Reference

• Parent tracker: Implementation Tracker: BookRunner Azure DevOps Migration #1
• Project brief target: Selenium + Jest + Supertest
• Local API contract: GROUP/BookRunner API Contract.md