feat: Add ActiveAdmin 4 and Propshaft support

- Switch to ActiveAdmin 4 beta with Tailwind CSS
- Replace Sprockets with Propshaft for Rails 8
- Migrate from jQuery to vanilla JavaScript
- Implement proper module initialization without setTimeout hacks
- Add esbuild for JavaScript bundling with clean import aliases
- Configure Tailwind CSS build process with ActiveAdmin plugin
- Update engine configuration for both Propshaft and Sprockets
- Add comprehensive documentation for AA4 gem updates
- Maintain backward compatibility with legacy AA versions

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

spec/dummy/config/initializers/active_admin.rb

@@ -216,6 +216,9 @@ ActiveAdmin.setup do |config|
   # To load a javascript file:
   #   config.register_javascript 'my_javascript.js'
 
+  # ActiveAdmin 4 with Propshaft loads assets via the application layout
+  # and the manifest.js file, not via register_stylesheet/register_javascript
+
   # == CSV options
   #
   # Set the CSV builder separator