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>

lib/activeadmin/quill_editor/engine.rb

@@ -11,16 +11,10 @@ module ActiveAdmin
       # No need for explicit asset path configuration with Propshaft
 
       initializer 'activeadmin_quill_editor.assets' do |app|
-        # For Propshaft (Rails 8 default)
-        if defined?(Propshaft)
+        # Add assets to precompile list for both Propshaft and Sprockets
+        if app.config.respond_to?(:assets)
           app.config.assets.precompile += %w[
-            activeadmin_quill_editor.js
-          ]
-        # For Sprockets (legacy support)
-        elsif app.config.respond_to?(:assets)
-          app.config.assets.precompile += %w[
-            activeadmin/quill_editor_input.js
-            activeadmin_quill_editor.js
+            activeadmin/quill_editor.js
           ]
         end
       end