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/config/manifest.js

@@ -1,3 +1,7 @@
 //= link_tree ../images
-//= link_directory ../stylesheets .css
+//= link_directory ../javascripts .js
+//= link active_admin.css
+//= link active_admin.js
+//= link activeadmin_quill_editor.js
+//= link application.css
 // OFF link active_storage_db_manifest.js