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>

spec/dummy/lib/tasks/active_admin.rake

@@ -0,0 +1,34 @@
+namespace :active_admin do
+  desc "Build Active Admin Tailwind stylesheets"
+  task build: :environment do
+    command = [
+      "npx", "tailwindcss",
+      "-i", Rails.root.join("app/assets/stylesheets/active_admin.css").to_s,
+      "-o", Rails.root.join("app/assets/builds/active_admin.css").to_s,
+      "-c", Rails.root.join("tailwind.config.js").to_s,
+      "-m"
+    ]
+
+    system(*command, exception: true)
+  end
+
+  desc "Watch Active Admin Tailwind stylesheets"
+  task watch: :environment do
+    command = [
+      "npx", "tailwindcss",
+      "--watch",
+      "-i", Rails.root.join("app/assets/stylesheets/active_admin.css").to_s,
+      "-o", Rails.root.join("app/assets/builds/active_admin.css").to_s,
+      "-c", Rails.root.join("tailwind.config.js").to_s,
+      "-m"
+    ]
+
+    system(*command)
+  end
+end
+
+Rake::Task["assets:precompile"].enhance(["active_admin:build"])
+
+Rake::Task["test:prepare"].enhance(["active_admin:build"]) if Rake::Task.task_defined?("test:prepare")
+Rake::Task["spec:prepare"].enhance(["active_admin:build"]) if Rake::Task.task_defined?("spec:prepare")
+Rake::Task["db:test:prepare"].enhance(["active_admin:build"]) if Rake::Task.task_defined?("db:test:prepare")