Wow, really great stuff! I wrote something similar for my high school a few years ago (back when I was less experienced with programming -- ack!). I used a library called JSCPP to interpret the C++ directly in the browser.

It included analogRead() and Serial functionality; maybe it can give you some ideas regarding the UI.

Here's the source: https://github.com/dpengineering/giffer-reborn

And you can try it here: https://dpengineering.github.io/giffer-reborn/