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/nomarchy/hooks/post-update.sample

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+# This hook is called after an Nomarchy system update has been performed.
+# To put it into use, remove .sample from the name.
+
+# Example: Show notification after the system has been updated.
+# notify-send -u low "Update Performed" "Your system is now up to date"