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>

docs/session-summary.md

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+# Session Summary - ActiveAdmin Quill Editor Modernization
+
+## Context
+We analyzed replacing Trumbowyg with Quill for ActiveAdmin 4.x and Rails 8 compatibility. After evaluating SunEditor (too large at 450KB after removing source maps) and Quill.js (~200KB), we chose to modernize the existing `activeadmin_quill_editor` gem.
+
+## Important Discoveries
+1. **Existing Test App**: The gem already has a full Rails test application at `spec/dummy/` (not using Combustion)
+2. **Existing Tests**: Comprehensive system tests already exist in `spec/system/`
+3. **Existing Formtastic Input**: `QuillEditorInput` class already exists, just needs jQuery removal
+
+## Key Decisions Made
+
+1. **Use Quill.js** instead of SunEditor
+   - Quill: ~200KB minified
+   - Clean delta-based architecture
+   - No jQuery dependency for the core library
+   - Existing gem foundation to build upon
+
+2. **Serve assets directly from vendor/assets**
+   - No NPM package needed initially
+   - Propshaft automatically serves from vendor/assets and app/assets
+   - Simple drop-in integration
+
+3. **Follow activeadmin_trumbowyg patterns**
+   - Combustion-based test app
+   - Modern CI with matrix testing (Ruby 3.3/3.4, Rails 7.x/8.x)
+   - Comprehensive GitHub Actions setup with caching
+   - ESBuild for test app assets
+
+## Files Created
+- `/data/activeadmin-quill_editor/docs/activeadmin-4-propshaft-update.md` - Complete migration guide
+- `/data/activeadmin-quill_editor/docs/session-summary.md` - This summary
+
+## Next Steps
+
+### Immediate Tasks
+1. Download Quill 2.x assets and place in vendor/assets
+2. Create new JavaScript initializer without jQuery
+3. Update Formtastic input class (already exists, just needs updates)
+4. Update existing spec/dummy test app for Propshaft/Rails 8
+5. Configure GitHub Actions CI/CD
+
+### Implementation Order
+1. **Asset Setup** (vendor/assets structure)
+2. **JavaScript Modernization** (remove jQuery)
+3. **Test App Updates** (spec/dummy with Propshaft)
+4. **CI/CD Setup** (GitHub Actions with matrix)
+5. **Documentation Update** (README, examples)
+
+### Testing Strategy
+- Use existing spec/dummy Rails app (no Combustion needed)
+- Update existing system tests, add missing cases from trumbowyg
+- Matrix test against Ruby 3.3/3.4
+- Test Rails 7.x and 8.x with ActiveAdmin 4.x beta
+- Consider adding Playwright for modern browser tests
+- SimpleCov for coverage with SonarQube integration
+
+## Technical Notes
+
+### Propshaft Asset Serving
+- Gems can provide assets in `vendor/assets` and `app/assets`
+- Propshaft automatically includes these paths in load path
+- No compilation needed for vendored assets
+- Digest stamping handled automatically
+
+### JavaScript API Changes
+Replace jQuery-based initialization:
+```javascript
+// Old (jQuery)
+$(document).ready(initQuillEditors);
+
+// New (Vanilla)
+document.addEventListener('DOMContentLoaded', initQuillEditors);
+document.addEventListener('turbo:load', initQuillEditors);
+```
+
+### Form Input Usage
+```ruby
+f.input :content, as: :quill_editor, input_html: {
+  data: {
+    options: {
+      theme: 'snow',
+      modules: { toolbar: [...] }
+    }
+  }
+}
+```
+
+## Resources
+- Quill.js 2.x: https://quilljs.com/
+- Propshaft docs: https://github.com/rails/propshaft
+- Reference gems:
+  - /data/activeadmin_trumbowyg (modern CI/test setup)
+  - /data/activeadmin-searchable_select (alternative patterns)
+
+## Contact for Questions
+Review the comprehensive guide at `activeadmin-4-propshaft-update.md` for complete implementation details.