The Send code or Login button looks greyed out / won't click. Why?

The buttons unlock as you go. Send code stays dimmed until you've typed a valid email address, and it also locks for a few seconds right after you click it — that's deliberate, so a double-click can't invalidate the code that's already on its way. Login stays dimmed until you've requested a code and typed one in. The code box itself only appears after you click Send code. Fill in the matching field (or wait out the brief lock) and the button becomes clickable.