Add Flutter client app for iOS, Android, Windows, MacOS, Linux Desktop

[thomastthai]

Having the web interface is good, but using Flutter to create one code base to work on iOS, Android, Windows, MacOS, and Linux Desktop would make it more convenient and flexible for more users.

[nicolasparada]

Would be nice to have more clients 😊
Personally I don't know flutter. I only do backend and some web frontend, but any contribution is very much welcome 😉

Currently to whole project is under a heavy refactor. So this would need to be on halt due to it.

[thomastthai]

I am also learning Flutter along with Go, so I will be able to help out later with the clients.