- Add GetToken() as unified auth entry point with method selection
- Prompt [1] OAuth / [2] API token on first launch, save choice
- Validate API tokens against /api/v1/users/self before saving
- Check saved API token validity on reuse; fallback to re-prompt on failure
- Add HTTP status check to GetCourseInfo and GetEnrolledCourses
- Remove --api flag, auth method is now persistent and interactive
- Add APIToken field to Credentials struct with omitempty JSON tag
- Implement GetAPIToken() method with persistent token storage
- Add SaveRefreshToken() helper to preserve API tokens during OAuth
- Implement --api CLI flag to use API token instead of OAuth
- Validate empty refresh tokens and skip refresh if invalid
- Update README with API token usage documentation
