feat(refactor): Implement 1A.1 Package Structure Refactoring

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.

internal/api/client/http_client.go

@@ -0,0 +1,11 @@
+package client
+
+import (
+	"io"
+	"net/url"
+)
+
+// HTTPClient defines the interface for HTTP operations
+type HTTPClient interface {
+	ConnectAPI(path string, method string, params url.Values, body io.Reader) ([]byte, error)
+}