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/tailwind-4/active_admin.rake

@@ -0,0 +1,55 @@
+namespace :active_admin do
+  desc 'Build Active Admin Tailwind stylesheets'
+  task :build do
+    require 'fileutils'
+
+    root = File.expand_path('../../', __dir__)
+
+    # Ensure builds directory exists
+    FileUtils.mkdir_p(File.join(root, 'app/assets/builds'))
+
+    # Build with Tailwind CLI
+    command = [
+      'npx', 'tailwindcss',
+      '-i', File.join(root, 'app/assets/stylesheets/active_admin.tailwind.css'),
+      '-o', File.join(root, 'app/assets/builds/active_admin.css'),
+      '-c', File.join(root, 'tailwind.config.js'),
+      '-m'
+    ]
+
+    system(*command, exception: true)
+
+    puts 'Built Active Admin CSS with Tailwind'
+  end
+
+  desc 'Watch Active Admin Tailwind stylesheets'
+  task :watch do
+    root = File.expand_path('../../', __dir__)
+
+    # Watch for changes
+    command = [
+      'npx', 'tailwindcss',
+      '--watch',
+      '-i', File.join(root, 'app/assets/stylesheets/active_admin.tailwind.css'),
+      '-o', File.join(root, 'app/assets/builds/active_admin.css'),
+      '-c', File.join(root, 'tailwind.config.js'),
+      '-m'
+    ]
+
+    system(*command)
+  end
+end
+
+# Enhance existing rake tasks
+if Rake::Task.task_defined?('assets:precompile')
+  Rake::Task['assets:precompile'].enhance(['active_admin:build'])
+end
+if Rake::Task.task_defined?('test:prepare')
+  Rake::Task['test:prepare'].enhance(['active_admin:build'])
+end
+if Rake::Task.task_defined?('spec:prepare')
+  Rake::Task['spec:prepare'].enhance(['active_admin:build'])
+end
+if Rake::Task.task_defined?('db:test:prepare')
+  Rake::Task['db:test:prepare'].enhance(['active_admin:build'])
+end