My diff tool, diff.so, is a web app, but it doesn’t send the text to the server (unless you publish). I wish there were a technical mechanism that could guarantee and enforce that, and certify it to users.
You could distribute it as a single index.html file, with unobfuscated/unminimized vanilla javascript. The user could then execute it offline no problem.
For fun, you could even use redbean ( https://justine.lol/redbean/index.html ), which has been discussed on here recently. That would give you a tiny executable running a webserver that serves your webapp locally. Probably not much advantage compared to a plain html file, though. Might be smaller overall, since everything would be compressed.