Implement a sample for MVVM

[viamgr]

The app needs a clear structure so it's easy to test and update. The best way is to use a simple plan called MVVM.

Here's what to do:

• [ ] Create a public thing called username that holds information and can be checked for changes.
• [ ] Make a thing called onSubmitClicked that does something when a button is clicked. It should also change another thing that remembers the click.
• [ ] Write tests that check if things work the right way.

By doing these steps, the app becomes easier to work with, and we can be sure it works as it should.