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/auth.go

@@ -0,0 +1,37 @@
+package client
+
+import (
+	"time"
+)
+
+// OAuth1Token represents OAuth 1.0a credentials
+type OAuth1Token struct {
+	Token       string
+	TokenSecret string
+	CreatedAt   time.Time
+}
+
+// Expired checks if token is expired (OAuth1 tokens typically don't expire but we'll implement for consistency)
+func (t *OAuth1Token) Expired() bool {
+	return false // OAuth1 tokens don't typically expire
+}
+
+// OAuth2Token represents OAuth 2.0 credentials
+type OAuth2Token struct {
+	AccessToken  string
+	RefreshToken string
+	TokenType    string
+	ExpiresIn    int
+	ExpiresAt    time.Time
+}
+
+// Expired checks if token is expired
+func (t *OAuth2Token) Expired() bool {
+	return time.Now().After(t.ExpiresAt)
+}
+
+// RefreshIfNeeded refreshes token if expired (implementation pending)
+func (t *OAuth2Token) RefreshIfNeeded(client *Client) error {
+	// Placeholder for token refresh logic
+	return nil
+}