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/javascript/active_admin.js

@@ -0,0 +1,26 @@
+// Import ActiveAdmin - this already includes all features and Rails UJS
+// DO NOT import Rails separately as it's already included and started in ActiveAdmin
+import '@activeadmin/activeadmin';
+
+// Import Quill from NPM and make it globally available
+import Quill from 'quill';
+window.Quill = Quill;
+
+// Import the image uploader plugin if available (optional)
+try {
+  const ImageUploader = require('quill-image-uploader');
+  window.ImageUploader = ImageUploader.default || ImageUploader;
+} catch(e) {
+  // Image uploader is optional (silent failure)
+}
+
+// Import the Quill Editor initialization module
+import QuillEditorModule from 'activeadmin_quill_editor';
+
+// Now that Quill is available, initialize the editors
+// This ensures proper initialization order without setTimeout hacks
+if (window.QuillEditor && window.QuillEditor.init) {
+  window.QuillEditor.init();
+} else if (QuillEditorModule && QuillEditorModule.init) {
+  QuillEditorModule.init();
+}