Enhancement: Set up unit testing framework (e.g., Jest or Mocha)

[3nln]

Enhancement Description

Currently, the repository lacks an automated testing framework. Running npm run test or npm test throws a missing script error. As the CLI tool grows, unit testing will be essential to catch regressions and ensure stability.

Proposed Solution

1. Install a test runner like jest, mocha, or vitest.
2. Add a test script in package.json.
3. Write basic test coverage for CLI commands (like lang, login, and status).
4. This will set the foundation for implementing GitHub Actions for CI pipeline in the future.