fix: Use mounted instead of context.mounted in LoginCard

[vargab95]

In my original PR, I've used mounted instead of context.mounted. However, it was merged as context.mounted checks. The problem is that, context.mounted has a hidden null check in the context getter of the State class. So an issue similar to #468 reappeared again in my crashlytics report. That's why I would like to replace the "context.mounted" checks in the LoginCard component with "mounted".