refactor: Remove jQuery dependency and consolidate JavaScript assets

- Rename vendor JS file to activeadmin/quill_editor.js to avoid conflicts
- Update import alias to use 'activeadmin/quill_editor' for consistency
- Remove jQuery-dependent legacy file (quill_editor_input.js)
- Simplify engine asset configuration - only one JS file now
- Update documentation with correct paths and import names
- All functionality preserved with vanilla JavaScript

BREAKING CHANGE: Drops support for jQuery-based initialization.
Applications using the legacy jQuery initialization will need to
update to the new module-based approach.

🤖 Generated with Claude Code

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

spec/dummy/app/javascript/active_admin.js

@@ -15,7 +15,7 @@ try {
 }
 
 // Import the Quill Editor initialization module
-import QuillEditorModule from 'activeadmin_quill_editor';
+import QuillEditorModule from 'activeadmin/quill_editor';
 
 // Now that Quill is available, initialize the editors
 // This ensures proper initialization order without setTimeout hacks