fix(admin): resolve hydration layout crash by removing layout-level return null and using mount-conditional text

frontend/src/app/admin/layout.tsx

@@ -33,10 +33,6 @@ export default function AdminLayout({
     return unsub;
   }, []);
 
-  if (!isMounted) {
-    return null;
-  }
-
   const verifyToken = useCallback(async (token: string): Promise<boolean> => {
     try {
       const response = await fetch(`${API_BASE}/api/v1/admin/verify`, {
@@ -82,7 +78,9 @@ export default function AdminLayout({
   if (isChecking && pathname !== "/admin/login") {
     return (
       <div className="min-h-screen bg-card flex items-center justify-center">
-        <div className="text-muted-foreground text-sm">{t('admin.layout.checking')}</div>
+        <div className="text-muted-foreground text-sm" suppressHydrationWarning>
+          {isMounted ? t('admin.layout.checking') : "Verifying authentication..."}
+        </div>
       </div>
     );
   }