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-show-done

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+# Display a "Done!" message with a spinner and wait for user to press any key.
+# Used by various install scripts to indicate completion.
+
+echo
+gum spin --spinner "globe" --title "Done! Press any key to close..." -- bash -c 'read -n 1 -s'