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

themes/templates/swayosd.css.tpl

@@ -0,0 +1,5 @@
+@define-color background-color {{ background }};
+@define-color border-color {{ foreground }};
+@define-color label {{ foreground }};
+@define-color image {{ foreground }};
+@define-color progress {{ accent }};