I have no idea how people aren’t getting this. People, if the website that the tool is hosted on is down or gone, then you are dead out of water. That’s why he made a local tool. Get it?
I tried React Native (only support JS on the client-side, need to build native modules to run some core functions for languages), Flutter (a few missing utilities, not so familiar).
I have answer why a desktop app in other comments so I won't repeat. Electron is the best fit in this situation.