This commit implements the package structure refactoring as outlined in phase1.md (Task 1A.1).
Key changes include:
- Reorganized packages into `pkg/garmin` for public API and `internal/` for internal implementations.
- Updated all import paths to reflect the new structure.
- Consolidated types and client logic into their respective new packages.
- Updated `cmd/garth/main.go` to use the new public API.
- Fixed various compilation and test issues encountered during the refactoring process.
- Converted `internal/api/client/auth_test.go` to a functional test.
This establishes a solid foundation for future enhancements and improves maintainability.
