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/navigation.js

@@ -4,6 +4,7 @@ import {
   getNavSections,
   getNavItems,
 } from './registry.js';
+import { isDevkitEnabled } from '../../shared/lib/appConfig.js';
 
 // Sections et items core — enregistrés à l'import de ce module.
 registerNavSection({ id: 'dashboard', title: 'Tableau de bord', icon: 'DashboardSquare03Icon', order: 10 });
@@ -14,6 +15,12 @@ registerNavItem({ id: 'users',     label: 'Utilisateurs',   icon: 'UserMultiple0
 registerNavItem({ id: 'roles',     label: 'Rôles',          icon: 'Crown03Icon',           href: '/admin/roles',     sectionId: 'system',    order: 20 });
 registerNavItem({ id: 'settings',  label: 'Paramètres',     icon: 'Settings02Icon',        href: '/admin/settings',  position: 'bottom',     order: 10 });
 
+if (isDevkitEnabled()) {
+  registerNavSection({ id: 'devkit', title: 'DevKit', icon: 'Wrench01Icon', order: 90 });
+  registerNavItem({ id: 'devkit-components', label: 'Composants', icon: 'Layers01Icon', href: '/admin/devkit/components', sectionId: 'devkit', order: 10 });
+  registerNavItem({ id: 'devkit-icons', label: 'Icônes', icon: 'Image01Icon', href: '/admin/devkit/icons', sectionId: 'devkit', order: 20 });
+}
+
 /**
  * Build sections for AdminSidebar. Items are sérialisables (pas de composants),
  * icônes en chaînes résolues côté client.