Feedback: I don't think the window should be resizeable on desktop
The app is actually the perfect size when you resize it to the smallest allowed size. Why not just force that size, as it doesn't really make sense to resize it. Sometimes the WM will maximize it too, which is weird. I really only see resizing it useful for adapting to mobile devices, so perhaps there should be a check for if the app is running on a phone, whereby it enables resizing for mobile only? I don't think being able to resize the window adds any meaningful value to the experience, and maybe detracts as now the user can use it in an impractical and aesthetically displeasing way.
As a side note, that's something I appreciate about macOS' calculator vs. GNOME's. Calculators really only make sense being used up to a certain size, so fullscreening a calculator on a widescreen desktop feels like a confusing waste of space. If you have to add useless padding to every edge of the app, I think it just doesn't need to be able to grow any bigger, and might just be more of a design burden than anything else. I think some things lose their elegance and utility when the user gets to resize them, and make more sense being hard-coded to a certain dimension when they are a small app like this, making them more utilitarian and purposeful. For instance, Kooha. Just feels unnecessary. Otherwise, your app is perfectly designed in my opinion!