Enhance OAuthHandler to improve CSRF protection and user creation logic

Commit notes

This commit updates the OAuthHandler by changing the SameSite attribute of the CSRF protection cookie from "Lax" to "Strict" and adding a Path attribute for better security. Additionally, it introduces a unique slug generation for new users during the Google callback process, ensuring that user slugs are unique in the database. These changes enhance security and improve user management within the application.