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.

pkg/garmin/client.go

@@ -0,0 +1,50 @@
+package garmin
+
+import (
+	internalClient "garmin-connect/internal/api/client"
+	"garmin-connect/internal/types"
+)
+
+// Client is the main Garmin Connect client type
+type Client struct {
+	Client *internalClient.Client
+}
+
+// NewClient creates a new Garmin Connect client
+func NewClient(domain string) (*Client, error) {
+	c, err := internalClient.NewClient(domain)
+	if err != nil {
+		return nil, err
+	}
+	return &Client{Client: c}, nil
+}
+
+// Login authenticates to Garmin Connect
+func (c *Client) Login(email, password string) error {
+	return c.Client.Login(email, password)
+}
+
+// LoadSession loads a session from a file
+func (c *Client) LoadSession(filename string) error {
+	return c.Client.LoadSession(filename)
+}
+
+// SaveSession saves the current session to a file
+func (c *Client) SaveSession(filename string) error {
+	return c.Client.SaveSession(filename)
+}
+
+// GetActivities retrieves recent activities
+func (c *Client) GetActivities(limit int) ([]Activity, error) {
+	return c.Client.GetActivities(limit)
+}
+
+// OAuth1Token returns the OAuth1 token
+func (c *Client) OAuth1Token() *types.OAuth1Token {
+	return c.Client.OAuth1Token
+}
+
+// OAuth2Token returns the OAuth2 token
+func (c *Client) OAuth2Token() *types.OAuth2Token {
+	return c.Client.OAuth2Token
+}