CalcPad is a different take on the calculator.
Runs on linux, mac, and windows.
Download the latest release for your platform.
There's a slightly simpler web version here.
It doesn't do any kind of file handling or http requests, as everything is stored using the browser's LocalStorage.
It works ok on phones, and you can install it as a PWA.
Since the app is fully offline, you can simply either download the latest built files, or build them yourself with npm build-web - the files will be in the docs/ directory.
Alternatively you can use shmup's redbean build.