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/font-set.sample

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+# This hook is called with the snake-cased name of the font that has just been set.
+# To put it into use, remove .sample from the name.
+
+# Example: Show the name of the theme that was just set.
+# notify-send -u low "New font" "Your new font is $1"