How do you know “it has no memory leaks, crashes, ANRs, no performance problems, no network latency bugs or anything” if you built it just yesterday? Isn’t it a bit too early for claims like this? I get it’s easy to bring ideas to life but aren’t we overly optimistic?

By tomorrow the app will be replaced with a new version from the other competitor, by that time the memory leak will not reveal itself

Part of the "one day" development time was exhaustively testing it. Since the tool's scope is so small, getting good test coverage was pretty easy. Of course, I'm not guaranteeing through formal verification methods that the code is bug free. I did find bugs, but they were all areas that were poorly specified by me in the requirements.