feat: add Google OAuth login

[rjwalters]

Summary

Add Google OAuth as a login option alongside the existing email/password authentication.

Motivation

Reduces friction for new users who don't want to create yet another email/password account. Google login is a widely expected option for web apps.

Requirements

• "Sign in with Google" button on the login page
• Google OAuth 2.0 flow (authorization code grant)
• Create account automatically on first Google login
• Link Google identity to existing account if email matches
• Session management consistent with existing cookie-based auth

Technical Notes

• Backend: Cloudflare Workers — needs OAuth token exchange in functions/api/auth/
• Frontend: LoginPage.tsx — add Google sign-in button
• Will need a Google Cloud project with OAuth 2.0 credentials configured
• Store Google OAuth client ID/secret as Cloudflare environment secrets