Add basic user authentication (optional)

[saviobatista]

Business Value
Authentication allows users to persist their history, keep documents private, and unlock future team-focused features.

Description
Add email/password or OAuth login with a simple account system. Store and load résumé versions tied to users.

Technical Details

• Use Supabase Auth or NextAuth
• Create /app/login, /app/register pages
• Associate saved sessions with user ID
• Gate version saving behind login (optional)

Acceptance Criteria

• Users can sign up and log in
• Saved versions are associated with user account
• Auth is optional for basic usage