You'll have to do in the server, if you're sane.

I bet Retool Forms does in the server.

You're worried the page will refresh with an empty form, if it fails. It's simple, just collect the data and return the form pre-filled using the value attribute, along with the error message.

If you don't want to refresh the page, you could even use something like htmx, which is a LOT simpler than React, but really not necessary for simple form submission.