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/engine/scripts/nomarchy-theme-update

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+for dir in ~/.config/nomarchy/themes/*/; do
+  if [[ -d $dir ]] && [[ ! -L ${dir%/} ]] && [[ -d $dir/.git ]]; then
+    echo "Updating: $(basename "$dir")"
+    git -C "$dir" pull
+  fi
+done