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/tailwind-active_admin.config.js

@@ -0,0 +1,18 @@
+const execSync = require("node:child_process").execSync;
+const activeAdminPath = execSync("bundle show activeadmin", {
+  encoding: "utf-8",
+}).trim();
+
+module.exports = {
+  content: [
+    `${activeAdminPath}/vendor/javascript/flowbite.js`,
+    `${activeAdminPath}/plugin.js`,
+    `${activeAdminPath}/app/views/**/*.{arb,erb,html,rb}`,
+    "./app/admin/**/*.{arb,erb,html,rb}",
+    "./app/views/active_admin/**/*.{arb,erb,html,rb}",
+    "./app/views/admin/**/*.{arb,erb,html,rb}",
+    "./app/views/layouts/active_admin*.{erb,html}",
+    "./app/assets/controllers/active_admin/**/*.js",
+  ],
+  darkMode: "selector"
+};