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/shared/lib/appConfig.js

@@ -48,3 +48,7 @@ export function getAppConfig() {
     currencySymbol: process.env.ZEN_CURRENCY_SYMBOL || '$',
   };
 }
+
+export function isDevkitEnabled() {
+  return process.env.ZEN_DEVKIT === 'true';
+}