Good stuff. A few things you should fix.

Having a Sign In With Google button with nothing else will discourage people from signing up. Most/lay people are wary about sharing Google credentials and thus you need to provide context/value proposition before they do.

Clicking any part of the input (and probably any part of the screen) should focus the input (the part where you write your todo). Currently you need to click on the middle of it for the text should focus.

Pressing enter should add the todo