Tool/app for learning category theory?
There are at least three encodings of (aspects of) Category Theory in Coq. Approaches using a proof assistant guarantee correctness by design, though I don't know whether they will enhance understand. You'll then have both the proof assistant and Category Theory to learn. 

If you want a reasonably introductory, computer science-oriented book, it's hard to beat Categories and Computer Science by R. F. C. Walters. His examples are mostly about data types and automata, rather than programming languages and lambdas. Conceptual Mathematics by Lawvere and Schanuel is also quite digestable. 

One approach combining these two is to use this Coq formalisation with Steve Awodey’s Book. The formalisation covers closely what is in the book. There's also a blog post about it.

That said, Category Theory is tough and its relationship with  programming quickly becomes quite distant, and so one needs to rely on one's mathematical background (which is now getting on to be 20 years old).