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

core/home/config/environment.d/fcitx.conf

@@ -0,0 +1,4 @@
+INPUT_METHOD=fcitx
+QT_IM_MODULE=fcitx
+XMODIFIERS=@im=fcitx
+SDL_IM_MODULE=fcitx