This looks amazing! I recently (5 days ago) started learning Japanese using Duolingo and have been taking hand written notes. I'm also thinking of creating a digital version using Zettlr[0] which I use daily. I'll also try out your app. If I may ask. What is the tech stack you used for building clients other than mobile as well as for mobile? Thanks.

[0] https://www.zettlr.com/