Move TUI logic to a new tui.go file, separating concerns and improving
maintainability. Integrate terminal-notifier to display a notification
when the timer completes on Darwin systems. Update README to include
terminal-notifier as a requirement.
- Updated dependencies in go.mod and go.sum to latest versions.
- Added README.md with usage instructions and requirements.
- Added justfile with build, run, and dev tasks.
- Updated .gitignore to exclude tmp/ directory.
