feat: implement comprehensive testing infrastructure

[renjfk]

Establish a complete testing framework for SimplyTrack to ensure code quality, reliability, and maintainability across all components.

Details

• Set up unit testing framework for core functionality
• Implement UI testing for user interactions and workflows
• Add code coverage reporting and quality gates
• Establish testing best practices and conventions
• Integrate testing into CI/CD pipeline
• Create test data fixtures and mocking utilities

This is a parent issue to organize and track testing-related sub-issues and improvements.