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-list

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+{
+  find ~/.config/nomarchy/themes/ -mindepth 1 -maxdepth 1 \( -type d -o -type l \) -printf '%f\n'
+  find "$NOMARCHY_PATH/assets/themes/" -mindepth 1 -maxdepth 1 -type d -printf '%f\n'
+} | sort -u | while read -r name; do
+  echo "$name" | sed -E 's/(^|-)([a-z])/\1\u\2/g; s/-/ /g'
+done