Add user authentication and multi-user support

[jeisaacs]

Summary

Introduce authentication so multiple users can securely access Dupl3x with personalized settings/history.

Why

• Current unauthenticated model is suitable only for trusted single-user setups
• Teams need isolated activity logs and saved preferences
• Provides foundation for future paid tiers or rate limits

Proposal

• Add optional auth (e.g., email+magic link, OAuth, or SSO) to the frontend and FastAPI backend
• Scope data models (settings, logs) by user account
• Provide migration path for existing single-user data

Acceptance Criteria

• Authentication layer can be enabled/disabled via config
• Users see only their own logs, settings, and files
• Session handling works across backend + frontend with secure storage
• Documentation covers setup along with security considerations