refactor: implement component-based architecture for enhanced maintainability

- Reorganize directory structure into core/, features/, and themes/
- Colocate application Nix logic, configs, scripts, and theme overrides
- Implement 'Inversion of Control' for theming: apps now pull theme-specific layouts
- Update flake.nix and shared library paths to match the new structure
- Document the new Feature-Centric architecture in README.md

features/desktop/waybar/themes/lumon/style.css

@@ -0,0 +1,2 @@
+@define-color foreground #d6e2ee;
+@define-color background #213442;