Thanks for your valuable feedback. Instrument list and temperament profiles would be expanded before adding other new features. The project started as a tuner app for South Indian Veena & as a fretting aid (based on ratios).
On base notes, have you tried using Just Intonation and other tuning profiles. They use perfect 5th. I will take a look at C# and Db difference in other temperaments. Maybe, our nomenclature needs a re-look.
Sorry, I don't only mean C# and Db, but just in general - the flat and sharp won't be the same outside of equal temperament. (I'm guessing details like that might matter to trombone players but I'm not sure; I play a C instrument myself).
I've only taken a cursory look and haven't tried playing violin with your app yet, but I'll post more feedback when I do!