Integrate SignIn and SignUp page with backend and fix flickering issue

[SajanGhuman]

This PR integrates the sign in and sign up pages with front end.

• The flickering caused by sign out and sign in now fixed
• The "api/auth/[...nextauth]/route.ts" now returns a much more structured user object. The return of user object is instantaneous and can be consumed by the auth wrapper to update auth state.
• The sign up page is also working as expected.