Add Google and Microsoft single sign-on to signup

User story As a new user from a company account, I want to sign up with my existing Google or Microsoft account, so that I don't have to create and remember another password.

Acceptance criteria

• Google Workspace and Microsoft Entra ID are both offered on the signup and login screens
• An existing email/password account can be linked to an SSO identity without losing data
• Failed handshakes show a readable error and are logged
• Signup completion rate is tracked separately for SSO vs email