Implement GarminUploader adapter around garminconnect library:
- Import activities using import_activity() not upload_activity()
- Treat duplicate activity responses as terminal success
- Raise GarminUploadBlocked for MFA to allow UI code collection
- Distinguish auth, transient, and other errors appropriately
- Helper functions for duplicate detection and activity ID extraction
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
