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/client_test.go

@@ -0,0 +1,41 @@
+package client_test
+
+import (
+	"net/http"
+	"net/url"
+	"testing"
+	"time"
+
+	"garmin-connect/internal/testutils"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"garmin-connect/internal/api/client"
+)
+
+func TestClient_GetUserProfile(t *testing.T) {
+	// Create mock server returning user profile
+	server := testutils.MockJSONResponse(http.StatusOK, `{
+		"userName": "testuser",
+		"displayName": "Test User",
+		"fullName": "Test User",
+		"location": "Test Location"
+	}`)
+	defer server.Close()
+
+	// Create client with test configuration
+	u, _ := url.Parse(server.URL)
+	c, err := client.NewClient(u.Host)
+	require.NoError(t, err)
+	c.HTTPClient = &http.Client{Timeout: 5 * time.Second}
+	c.AuthToken = "Bearer testtoken"
+
+	// Get user profile
+	profile, err := c.GetUserProfile()
+
+	// Verify response
+	require.NoError(t, err)
+	assert.Equal(t, "testuser", profile.UserName)
+	assert.Equal(t, "Test User", profile.DisplayName)
+}