An official cross platform syncthing app for desktop and mobile.
I would suggest using Flutter or Tauri (I do think flutter is the better option). Both allow making an app with a single codebase and compiling the app for desktop, android or ios.
What is your use case for this? What does this solve that the web UI does not?
There will be a native app on the system instead of using a Web UI which allows deeper integration with the system such as tray icons on desktop, notification on mobile.
There are third party tray icon options out there. And mobile versions, that I believe support notifications.