They have web app but had terrible experience with it (can't find devices when you are using the mobile app and the other device is using the web app).
Your question doesn't make sense, GrapheneOS said that they prefer spending their time implementing and fixing security issues rather than things that can be easily changed from open source offerings. See https://news.ycombinator.com/item?id=47247778.
https://github.com/localsend/web