GitHub OAuth support

WriftAI now supports signing in with GitHub. Authenticate using your existing GitHub account — no new password to manage.

If you already have a WriftAI account, your GitHub account will be linked automatically when you sign in for the first time using the same email address. Email and password sign-in remains available.

GitHub is a natural fit as our first OAuth provider — most WriftAI users already have a GitHub account as part of their development workflow.