Add Google OAuth Login Support (Backend Google token verification + Frontend GSI button and flow)

[puneeth-webdev218]

Added Google Identity Services (GSI) login button to the frontend login page

Implemented Google token verification using Google Auth Library on backend

Added /auth/google API route for Google login

If a Google user exists → return JWT + user object

If not → automatically create a new user record (no password required)

Added isGoogleUser and profilePicture fields to User model

Ensured Google-authenticated users can access all existing protected routes

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