- auth.ts: add google/github social providers from better-auth/social-providers
- auth.ts: add getActiveProviders() to enumerate configured OAuth/social providers
- index.ts: add /api/auth/providers public endpoint for frontend
- App.tsx: update LoginPage to show Google/GitHub buttons based on /api/auth/providers response
Co-Authored-By: Paperclip <noreply@paperclip.ing>