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/index.js

@@ -27,7 +27,7 @@ export * as pdf from "./core/pdf/index.js";
 // Do not export here to avoid mixing client/server boundaries
 
 // Export app configuration utilities
-export { getAppName, getAppConfig, getSessionCookieName, getPublicBaseUrl } from "./shared/lib/appConfig.js";
+export { getAppName, getAppConfig, getSessionCookieName, getPublicBaseUrl, isDevkitEnabled } from "./shared/lib/appConfig.js";
 
 // Export initialization utilities
 export { initializeZen, resetZenInitialization } from "./shared/lib/init.js";