Is it realistic to think I can build and app for Desktop and Mobile and have them share much of the same UI code?
So far I haven't had much luck with webview in mobile – I can't even get it to load images.
Much of the non UI code should probably work on both platforms. The UI paradigms are currently different between Desktop and Mobile. Also, given this disparity in screen size, connectivity, memory, and storage between the platforms, your best bet is to treat the two separately so that you can provide the best experience for your users.
In other words, I want to have a Desktop app and a Mobile app share the exact same library source files. Is there a good method of doing that, other than having multiple copies of those library files – one in each project?