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/battery-low.sample

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+# This hook is called with the current battery percentage when the low battery
+# notification is sent. To put it into use, remove .sample from the name.
+
+SOUND_FILE="/usr/share/sounds/freedesktop/stereo/dialog-warning.oga"
+
+if nomarchy-cmd-present mpv && [[ -f $SOUND_FILE ]]; then
+  mpv --no-video "$SOUND_FILE" >/dev/null 2>&1
+fi