Enhance PNPM configuration and update dependencies for frontend

Commit notes

- Introduce a new .pnpmrc file for improved Netlify compatibility with shamefully-hoist settings. - Update various dependencies in pnpm-lock.yaml, including @mantine packages and react-router to version 7.5.2 for better performance and stability. - Adjust frontend/netlify.toml to specify the correct publish directory and add NPM_FLAGS for build optimization. - Modify frontend/package.json to reflect updated react-router versions and ensure compatibility with the latest features.

This update streamlines the frontend setup and enhances compatibility with Netlify and React Router.