This won’t work because there is actual server-side code running
That is not true! You can figure that out for yourself - open up the site, disconnect your internet and resize/crop some images. It will do it just fine, because all of that code runs in your own browser.
If Photopea was so simple, you could just download the necessary parts and self-host.
This won’t work because there is actual server-side code running, meaning you’re hogging someone else’s resources to do your commercial-grade tasks.
That is not true! You can figure that out for yourself - open up the site, disconnect your internet and resize/crop some images. It will do it just fine, because all of that code runs in your own browser.