What is the state of the art architecture for an application that will have both web and desktop frontends?

I've been planning to build my own personal library application, but since I want it to run both on the web and on the desktop (and perhaps on Mobile in the future), I was stuck on choosing what is the best approch to adopt.

I've considered adopting a Microservices approach (or a mono-service, for that matter, since it's not a big application) and accessing it using RESTful techniques. But I don't know how it could be exposed to a desktop front-end in a elegant, scalable manner, because I don't wanna have different APIs for different frontends.

Is there any recommended pattern to follow in this case? Are there open source examples upon which I could base my case?

PS.: regarding the technology, I'm going with either .NET (preferably) or Java.