Entering a credit card number incorrectly puts things in a weird state

[ItsHarper]

Description

When entering payment information for a class that has a fee paid through the calendar, if the form is submitted with an internally-invalid credit card number (one that doesn't check out using Luhn's algorithm), the front end seems to see that and immediately inform the user of the problem, but it seems like the request to the server still gets sent, and the page gets stuck loading indefinitely. I had to refresh, and lose all of the information I had entered (which was especially infuriating because of #197).

I've only entered the number incorrectly once, and have not tried to reproduce the problem.

Smartphone information

• Device: Pixel 6
• OS: Android 15
• Browser: Chrome 137