Looks good! I've been thinking about building something similar but as a desktop app (and maybe browser extension) which would work with whatever text you have on the screen. It seems doable by (ab)using the OS accessibility APIs. I find it hard to stick to importing text, reading them in the app, and marking the words. Having something which works in the background and can tell you where you've previously seen words…
Are you familiar with Language Reactor or Migaku? I think there are a couple others too. They're all implemented as browser extensions, but that works out pretty well because most content that's useful for language learning gets accessed through a browser these days, anyway.