docs: add comprehensive testing strategy

Add detailed testing strategy document covering:
- Testing goals and TDD principles
- Testing pyramid with 80%+ coverage target
- Unit, component, integration, and E2E testing strategies
- Tool setup (Vitest, React Testing Library, Playwright)
- Mocking strategies and best practices
- CI/CD integration guidelines
- Implementation roadmap with 7 phases

Target: 80%+ overall coverage, 100% for critical paths
Priority: Component tests (React Testing Library) and key E2E workflows
Scope: Chrome only, no cross-browser automated testing

Author: claude