fix: update isCurrentDirRoot check to use .next directory

apps/app/src/server/util/project-dir-utils.ts

@@ -3,9 +3,7 @@ import path from 'node:path';
 import process from 'node:process';
 import { isServer } from '@growi/core/dist/utils/browser-utils';
 
-const isCurrentDirRoot =
-  isServer() &&
-  (fs.existsSync('./next.config.ts') || fs.existsSync('./next.config.js'));
+const isCurrentDirRoot = isServer() && fs.existsSync('./.next');
 
 export const projectRoot = isCurrentDirRoot
   ? process.cwd()