Add basic testing

[BrayanMQ]

Configure and add basic tests for critical components and functions.

Tasks

1. Install testing dependencies (Jest, React Testing Library)
2. Configure Jest for Next.js
3. Create tests for: streak calculation functions, statistics functions, critical components (HabitCard, AddHabitModal)
4. Basic integration tests for main flows
5. Configure CI to run tests (optional)

Acceptance Criteria

• Tests configured and working
• Critical tests implemented
• Tests pass correctly
• Reasonable coverage