Add validation of number input (to c1, possibly other pages)

`parseFloat()` is being used to parse the number input on the c1 step, which may not end up with the right result because of how permissive this function is. Instead, we should have a front end validation to get the user to input numbers only.

There may be other inputs where the same thing is happening, so might be a good idea to audit the app for these and use the validation on all of them.