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/auth/credentials/credentials.go

@@ -0,0 +1,32 @@
+package credentials
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/joho/godotenv"
+)
+
+// LoadEnvCredentials loads credentials from .env file
+func LoadEnvCredentials() (email, password, domain string, err error) {
+	// Load .env file
+	if err := godotenv.Load(); err != nil {
+		return "", "", "", fmt.Errorf("error loading .env file: %w", err)
+	}
+
+	email = os.Getenv("GARMIN_EMAIL")
+	password = os.Getenv("GARMIN_PASSWORD")
+	domain = os.Getenv("GARMIN_DOMAIN")
+
+	if email == "" {
+		return "", "", "", fmt.Errorf("GARMIN_EMAIL not found in .env file")
+	}
+	if password == "" {
+		return "", "", "", fmt.Errorf("GARMIN_PASSWORD not found in .env file")
+	}
+	if domain == "" {
+		domain = "garmin.com" // default value
+	}
+
+	return email, password, domain, nil
+}