Objectify is a nice framework, but after reading the examples in README made me feel like I am required to take extra iterations: create new responder files, write more codes for each Responder class, etc. This approach is little tedious.
I personally think that writing reusable libraries using Decorator/Presenter patterns or Modules with ActiveSupport::Concern is simple approach.
Was going to pledge the project, but saw "it is browser-based, it will packaged in a webview of some kind so that it feels like a normal app". Nothing wrong with browser-based editor, but I am comfortable with native app. That's my personal preference.
BTW, it would be nice to have the ability to delete link and re-enter the link because I have modified details @ gumroad and then your site is still saved old information.