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/data/base_test.go

@@ -0,0 +1,74 @@
+package data
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"garmin-connect/internal/api/client"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// MockData implements Data interface for testing
+type MockData struct {
+	BaseData
+}
+
+// MockClient simulates API client for tests
+type MockClient struct{}
+
+func (mc *MockClient) Get(endpoint string) (interface{}, error) {
+	if endpoint == "error" {
+		return nil, errors.New("mock API error")
+	}
+	return "data for " + endpoint, nil
+}
+
+func TestBaseData_List(t *testing.T) {
+	// Setup mock data type
+	mockData := &MockData{}
+	mockData.GetFunc = func(day time.Time, c *client.Client) (interface{}, error) {
+		return "data for " + day.Format("2006-01-02"), nil
+	}
+
+	// Test parameters
+	end := time.Date(2023, 6, 15, 0, 0, 0, 0, time.UTC)
+	days := 5
+	c := &client.Client{}
+	maxWorkers := 3
+
+	// Execute
+	results, errs := mockData.List(end, days, c, maxWorkers)
+
+	// Verify
+	assert.Empty(t, errs)
+	assert.Len(t, results, days)
+	assert.Contains(t, results, "data for 2023-06-15")
+	assert.Contains(t, results, "data for 2023-06-11")
+}
+
+func TestBaseData_List_ErrorHandling(t *testing.T) {
+	// Setup mock data type that returns error on specific date
+	mockData := &MockData{}
+	mockData.GetFunc = func(day time.Time, c *client.Client) (interface{}, error) {
+		if day.Day() == 13 {
+			return nil, errors.New("bad luck day")
+		}
+		return "data for " + day.Format("2006-01-02"), nil
+	}
+
+	// Test parameters
+	end := time.Date(2023, 6, 15, 0, 0, 0, 0, time.UTC)
+	days := 5
+	c := &client.Client{}
+	maxWorkers := 2
+
+	// Execute
+	results, errs := mockData.List(end, days, c, maxWorkers)
+
+	// Verify
+	assert.Len(t, errs, 1)
+	assert.Equal(t, "bad luck day", errs[0].Error())
+	assert.Len(t, results, 4) // Should have results for non-error days
+}