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/AdminPage.server.js

@@ -2,12 +2,14 @@ import AdminPageClient from './AdminPage.client.js';
 import { protectAdmin } from './protect.js';
 import { collectWidgetData } from './registry.js';
 import { getAppConfig, getPublicBaseUrl } from '@zen/core';
+import { isDevkitEnabled } from '../../shared/lib/appConfig.js';
 
 export default async function AdminPage({ params }) {
   const resolvedParams = await params;
   const session = await protectAdmin();
   const widgetData = await collectWidgetData();
   const appConfig = { ...getAppConfig(), siteUrl: getPublicBaseUrl() };
+  const devkitEnabled = isDevkitEnabled();
 
   return (
     <AdminPageClient
@@ -15,6 +17,7 @@ export default async function AdminPage({ params }) {
       user={session.user}
       widgetData={widgetData}
       appConfig={appConfig}
+      devkitEnabled={devkitEnabled}
     />
   );
 }