feat(admin): add devkit developer tools section

- add `ZEN_DEVKIT` env variable to enable/disable devkit
- add `isDevkitEnabled()` utility and export it from public api
- register devkit nav section and items conditionally when devkit is enabled
- add devkit route handling in admin page client and server
- add DevkitPage, ComponentsPage, and IconsPage client components

src/features/admin/devkit/DevkitPage.client.js

@@ -0,0 +1,23 @@
+'use client';
+
+import { registerPage } from '../registry.js';
+import ComponentsPage from './ComponentsPage.client.js';
+import IconsPage from './IconsPage.client.js';
+
+function DevkitPage({ params, devkitEnabled }) {
+  if (!devkitEnabled) {
+    return (
+      <div className="flex items-center justify-center py-24 text-neutral-400 dark:text-neutral-600 text-sm">
+        DevKit désactivé. Définir <code className="mx-1 font-mono bg-neutral-100 dark:bg-neutral-800 px-1 rounded">ZEN_DEVKIT=true</code> pour activer.
+      </div>
+    );
+  }
+
+  const subPage = params?.[1] || 'components';
+
+  if (subPage === 'icons') return <IconsPage />;
+  return <ComponentsPage />;
+}
+
+export default DevkitPage;
+registerPage({ slug: 'devkit', title: 'DevKit', Component: DevkitPage });