Introduce Google Sign In Button on the Platform

🔍 Problem Statement As a Pelcro platform user, I must create and remember a separate password to sign in, which adds friction at login and increases account abandonment and password-reset support volume.

💡 User Story As a Pelcro platform user, I want to sign in with my Google account, so that I can access the platform without managing a separate Pelcro password.

🎯 Definition of Done (DoD)

✔ Given a user on the Pelcro login screen, when they click "Sign in with Google" and complete the Google OAuth 2.0 consent flow, then they are authenticated into Pelcro and a session is issued.

✔ Given a Google sign-in for an email that matches an existing Pelcro account, when authentication succeeds, then the Google identity is linked to that account and the user is signed in without creating a duplicate.

✔ Given a Google sign-in for an email with no existing Pelcro account, when authentication succeeds, then a new Pelcro user is provisioned using the verified Google email, first name, and last name.

✔ This change will impact UI, API, and SDK, specifically the login Element in ReactJS Elements, a new /auth/google (or equivalent) endpoint on the Laravel backend, and a corresponding method in the JS-SDK.

✔ This solution will include the following limitations: Google as the only IdP in this scope; no account unlinking UI; no admin-side login via Google; no SSO/SAML; no migration of existing password-only users to Google-only.