When a user has set up their google account, has an active session, and lands on a page other than / they are redirected to / where it then detects that they are signed in and redirects them to /level/1.

Desired Behavior is that they stay on the page they land on. Or, at the very least, are redirected to the page they landed on.

When clicking quickly, it is easy to accidentally select some text on the calculator. This is annoying, bad UI.

Buttons that change value over time. So, the first time you push the button it is +1 then +2 then +3...

Buttons with hidden functions. So users have to deduce what the function is. Will be most fun if you play with this where start value is 2 and the output of any of the buttons is 4 because 2 + 2 = 4, 2 * 2 = 4 and 2 ^ 2 = 4.

Should be transparent

If a user does not have a profile image, there should be a fallback.

It should be simple and clear enough that a user should understand. Maybe some difficulty value.

+3 *5 that happen in a single press.

So, if the start is 2 and your press the above button it would land on 25 but would not register as having passed through 5.

When divide by 3, rounding error occurs. Adjusting the precision may fix this.