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

features/apps/scripts/nomarchy-webapp-handler-hey

@@ -0,0 +1,11 @@
+#!/bin/bash
+url="$1"
+web_url="https://app.hey.com"
+
+# Handle mailto: URLs
+if [[ $url =~ ^mailto: ]]; then
+  email=$(echo "$url" | sed 's/mailto://')
+  web_url="https://app.hey.com/messages/new?to=$email"
+fi
+
+exec nomarchy-launch-webapp "$web_url"