Add Login/signup to headless storefront using graphql.

[hirenpatel]

i am using shopify as backend. use headless storefront api with graphql. i want to add login/signup in my storefront header. 
so how can i do it and when when user login/signup it makes query in our shopify backend.
once i login and redirect to checkout that is shopify's checkout page i am not again ask for login if i already logged in from storefront.