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/app/assets/javascripts/activeadmin_quill_editor.js

@@ -0,0 +1 @@
+../../../../../vendor/assets/javascripts/activeadmin_quill_editor.js

spec/dummy/app/assets/javascripts/application.js

@@ -0,0 +1,2 @@
+//= require_tree ../builds
+//= require activeadmin_quill_editor