Release v4.0.0

What's new in v4.0.0

• Modern async/threaded architecture for all heavy operations (speedtest, network info, plotting)
• Beautiful dark UI with ttkbootstrap
• Toast notifications, smooth progress bars, improved UX
• Modular, testable, and robust codebase
• All comments and docstrings in English
• Full PyInstaller support for macOS .app bundle
• Clean .gitignore, all cache and local files excluded from repo
• 100% passing unit tests and Pylint exceptions for CI