closes#112
Replace `os.UserConfigDir()` with explicit `~/.config` fallback to align
macOS, Linux, and other platforms on same config location
Respects `XDG_CONFIG_HOME` if set, otherwise uses `~/.config/iris/`
This project is starting to grow, and I've seen flaws in the initial
project structure design
I think I'll restructure the entire project directory for better
scalability and also cleaner