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/system/scripts/nomarchy-restart-xcompose

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+# Restart the XCompose input method service (fcitx5) to apply new compose key settings.
+
+nomarchy-restart-app fcitx5 --disable notificationitem