fix: Allow ActiveAdmin 4.x beta versions in gemspec dependency

Update activeadmin dependency from '>= 4.0' to '>= 4.0.0.beta'
to allow pre-release beta versions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

activeadmin-quill_editor.gemspec

@@ -25,6 +25,6 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['{app,lib,vendor}/**/*', 'LICENSE.txt', 'Rakefile', 'README.md']
   spec.require_paths = ['lib']
 
-  spec.add_dependency 'activeadmin', '>= 4.0', '< 5'
+  spec.add_dependency 'activeadmin', '>= 4.0.0.beta', '< 5'
   spec.add_dependency 'rails', '>= 7.2'
 end