Implemented a robust testing infrastructure to improve code quality and maintainability.

- Set up Vitest with multi-environment support (main, renderer, integration)
- Added test helpers and utilities for mocking Electron APIs
- Created example tests for database manager and PostgreSQL implementation
- Configured coverage reporting with @vitest/coverage-v8
- Added GitHub Actions workflow for automated testing on PRs
- Included React Testing Library for component testing
- Documented testing patterns and best practices

Current status:
- 14 tests passing (DatabaseManager fully tested)
- Test framework operational with 11.5% initial coverage
- Ready for community contributions to expand test coverage

Added robust integration testing infrastructure for real database testing with Docker support.

- Created Docker Compose setup for PostgreSQL and ClickHouse test containers
- Implemented test helpers for table management and transactions
- Added comprehensive bulk operations integration tests
- Included test isolation with automatic cleanup
- Added npm scripts for easy Docker-based testing
- Documented integration testing patterns and best practices

Integration tests can now:
- Test against real databases using Docker containers
- Handle test data isolation and cleanup
- Verify bulk operations and transactions
- Skip gracefully when databases are unavailable

Removed hybrid approach and consolidated on Testcontainers for cleaner, more maintainable code.

- Removed Docker Compose setup and related files
- Cleaned up test code by removing unnecessary comments
- Simplified helper functions and test structure
- Consolidated integration tests into single database.test.ts
- Updated documentation to reflect single approach
- Added PostgreSQL and ClickHouse container helpers
- Implemented automatic container lifecycle management

Benefits:
- No manual setup required
- True test isolation with fresh containers
- Dynamic port allocation prevents conflicts
- Automatic cleanup guaranteed
- CI/CD ready without additional configuration