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/auth/index.js

@@ -1,10 +1,11 @@
 /**
  * Zen Authentication — server barrel (Next.js-free).
  *
- * Does NOT re-export actions.js — that file imports `next/headers` at the
- * top level and cannot be pulled in via this barrel (which is imported by
- * external modules during instrumentation, before Next.js aliases are active).
- * Import server actions explicitly via @zen/core/features/auth/actions.
+ * Ne re-exporte PAS actions.js — ce fichier importe `next/headers` au niveau
+ * top-level et ne peut pas être tiré via ce barrel (qui peut être importé par
+ * des modules externes pendant leur register(), avant que Next.js ait activé
+ * ses alias de modules). Importer les server actions via
+ * @zen/core/features/auth/actions.
  */
 
 export {