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.client.js

@@ -8,8 +8,9 @@ import './pages/ProfilePage.client.js';
 import './pages/SettingsPage.client.js';
 import './pages/ConfirmEmailChangePage.client.js';
 import './widgets/index.client.js';
+import './devkit/DevkitPage.client.js';
 
-export default function AdminPageClient({ params, user, widgetData, appConfig }) {
+export default function AdminPageClient({ params, user, widgetData, appConfig, devkitEnabled }) {
   const parts = params?.admin || [];
   const [first] = parts;
 
@@ -25,5 +26,8 @@ export default function AdminPageClient({ params, user, widgetData, appConfig })
   if (slug === 'settings') {
     return <Component user={user} appConfig={appConfig} />;
   }
+  if (slug === 'devkit') {
+    return <Component user={user} params={parts} devkitEnabled={devkitEnabled} />;
+  }
   return <Component user={user} params={parts} />;
 }