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>

lib/activeadmin/quill_editor/engine.rb

@@ -6,6 +6,30 @@ module ActiveAdmin
   module QuillEditor
     class Engine < ::Rails::Engine
       engine_name 'activeadmin_quill_editor'
+
+      # Propshaft automatically includes vendor/assets and app/assets paths
+      # No need for explicit asset path configuration with Propshaft
+
+      initializer 'activeadmin_quill_editor.assets' do |app|
+        # For Propshaft (Rails 8 default)
+        if defined?(Propshaft)
+          app.config.assets.precompile += %w[
+            activeadmin_quill_editor.js
+          ]
+        # For Sprockets (legacy support)
+        elsif app.config.respond_to?(:assets)
+          app.config.assets.precompile += %w[
+            activeadmin/quill_editor_input.js
+            activeadmin_quill_editor.js
+          ]
+        end
+      end
+
+      initializer 'activeadmin_quill_editor.formtastic' do
+        ActiveSupport.on_load :active_admin do
+          require 'formtastic/inputs/quill_editor_input'
+        end
+      end
     end
   end
-end
+end