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/esbuild.config.js

@@ -0,0 +1,50 @@
+#!/usr/bin/env node
+const esbuild = require('esbuild');
+const path = require('path');
+const { execSync } = require('node:child_process');
+
+// For development, use the local gem path
+const gemPath = path.resolve(__dirname, '../..');
+
+// Configuration for esbuild with proper module resolution
+const config = {
+  entryPoints: ['app/javascript/active_admin.js'],
+  bundle: true,
+  sourcemap: true,
+  format: 'iife',
+  outdir: 'app/assets/builds',
+  publicPath: '/assets',
+  loader: {
+    '.js': 'js',
+  },
+  // Define global Quill for the initialization script
+  define: {
+    'global': 'window'
+  },
+  // Use alias for clean imports
+  alias: {
+    'activeadmin_quill_editor': path.join(gemPath, 'vendor/assets/javascripts/activeadmin_quill_editor.js')
+  }
+};
+
+// Check if we're in watch mode
+const watchMode = process.argv.includes('--watch');
+
+if (watchMode) {
+  // Start the build with watch mode
+  esbuild.context(config).then(ctx => {
+    ctx.watch();
+    console.log('Watching for changes...');
+  }).catch(error => {
+    console.error('Build failed:', error);
+    process.exit(1);
+  });
+} else {
+  // Single build
+  esbuild.build(config).then(() => {
+    console.log('Build completed');
+  }).catch(error => {
+    console.error('Build failed:', error);
+    process.exit(1);
+  });
+}