feat: various fixes for form inputs and /register/social MP-938

[emuvente]

I found various issues when completing some checkouts in dev, one using a guest account and another using a new signup.

• form input error messages were showing when they shouldn't. I found all the cases where checks for $error had been switched to $invalid and switched them back.
• the first name input was always displaying on /register/social, even when it was not required. looks like that was due to a line for debugging that was left in.
• it's possible to click "complete registration" multiple times, but only the first request will succeed. Clicking the button cancels the in-flight request and starts a new one, which will end in being sent to the error page. I added a loading overlay so that none of the form can be interacted with after the button is clicked.
• I found a case of KvBaseInput usage where classes were still being applied directly instead of to a wrapper
• I replaced the loading spinner in the loading overlay with the one from kv-components:

Screen.Recording.2024-10-16.at.2.04.54.PM.mov