fix: Replace isLoggedIn with token/user check in board pages

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/app/board/page.tsx

@@ -10,7 +10,8 @@ import { useTranslate } from '@/lib/useTranslate';
 export default function BoardPage() {
   const router = useRouter();
   const searchParams = useSearchParams();
-  const { user, isLoggedIn } = useAuthStore();
+  const { user, token } = useAuthStore();
+  const isLoggedIn = !!token && !!user;
   const { translate, language } = useTranslate();
 
   const [posts, setPosts] = useState<BoardPostListItem[]>([]);