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.tailwind.css

@@ -0,0 +1,16 @@
+@import "tailwindcss/base";
+@import "tailwindcss/components";
+@import "tailwindcss/utilities";
+
+@import "activeadmin-tom_select/src/tom-select-tailwind.css";
+
+/* Additional batch actions dropdown styles that aren't in the ActiveAdmin plugin */
+@layer components {
+  .batch-actions-dropdown-toggle:disabled {
+    @apply opacity-50 cursor-not-allowed;
+  }
+
+  .batch-actions-dropdown-arrow {
+    @apply w-2.5 h-2.5;
+  }
+}