This commit includes the remaining files from the authentication flow refactoring.\nThese changes were part of the initial diff between c00ea67f31 and HEAD,\nand complete the transition to the new SSO and OAuth-based authentication mechanism.
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.
