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/types/auth.go

@@ -0,0 +1,28 @@
+package types
+
+import "time"
+
+// OAuthConsumer represents OAuth consumer credentials
+type OAuthConsumer struct {
+	ConsumerKey    string `json:"consumer_key"`
+	ConsumerSecret string `json:"consumer_secret"`
+}
+
+// OAuth1Token represents OAuth1 token response
+type OAuth1Token struct {
+	OAuthToken       string `json:"oauth_token"`
+	OAuthTokenSecret string `json:"oauth_token_secret"`
+	MFAToken         string `json:"mfa_token,omitempty"`
+	Domain           string `json:"domain"`
+}
+
+// OAuth2Token represents OAuth2 token response
+type OAuth2Token struct {
+	AccessToken  string    `json:"access_token"`
+	TokenType    string    `json:"token_type"`
+	ExpiresIn    int       `json:"expires_in"`
+	RefreshToken string    `json:"refresh_token"`
+	Scope        string    `json:"scope"`
+	CreatedAt    time.Time // Used for expiration tracking
+	ExpiresAt    time.Time // Computed expiration time
+}