refactor(admin): split protect guards into dedicated export path

- remove `protectAdmin`/`isAdmin` re-exports from `features/admin/index.js` to avoid top-level `next/headers` import
- add `./features/admin/protect` export entry in `package.json`
- lazy-import `next/headers` in `router.js` `requireAuth` to defer resolution
- update `features/admin/README.md` to document new import paths
- translate `features/auth/index.js` comment to French for consistency

src/features/admin/README.md

@@ -10,7 +10,7 @@ Ce répertoire fournit l'interface d'administration complète : layout, navigati
 
 ```
 src/features/admin/
-├── index.js                    protectAdmin, isAdmin, buildNavigationSections, registre
+├── index.js                    buildNavigationSections, registre (Next.js-free)
 ├── protect.js                  gardes d'accès
 ├── navigation.js               buildNavigationSections, buildBottomNavItems
 ├── registry.js                 registre runtime d'extensions
@@ -50,7 +50,11 @@ src/features/admin/
 ## Import
 
 ```js
-import { protectAdmin, isAdmin, buildNavigationSections } from '@zen/core/features/admin';
+// Gardes RSC — chemin dédié (next/navigation + next/headers, non compatible turbopackIgnore)
+import { protectAdmin, isAdmin } from '@zen/core/features/admin/protect';
+
+// Registre et navigation — compatible modules externes
+import { buildNavigationSections } from '@zen/core/features/admin';
 import {
   registerWidget,
   registerWidgetFetcher,