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/stats/base.go

@@ -0,0 +1,101 @@
+package stats
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+	"time"
+
+	"garmin-connect/internal/api/client"
+	"garmin-connect/internal/utils"
+)
+
+type Stats interface {
+	List(end time.Time, period int, client *client.Client) ([]interface{}, error)
+}
+
+type BaseStats struct {
+	Path     string
+	PageSize int
+}
+
+func (b *BaseStats) List(end time.Time, period int, client *client.Client) ([]interface{}, error) {
+	endDate := utils.FormatEndDate(end)
+	var allData []interface{}
+	var errs []error
+
+	for period > 0 {
+		pageSize := b.PageSize
+		if period < pageSize {
+			pageSize = period
+		}
+
+		page, err := b.fetchPage(endDate, pageSize, client)
+		if err != nil {
+			errs = append(errs, err)
+			// Continue to next page even if current fails
+		} else {
+			allData = append(page, allData...)
+		}
+
+		// Move to previous page
+		endDate = endDate.AddDate(0, 0, -pageSize)
+		period -= pageSize
+	}
+
+	// Return partial data with aggregated errors
+	var finalErr error
+	if len(errs) > 0 {
+		finalErr = fmt.Errorf("partial failure: %v", errs)
+	}
+	return allData, finalErr
+}
+
+func (b *BaseStats) fetchPage(end time.Time, period int, client *client.Client) ([]interface{}, error) {
+	var start time.Time
+	var path string
+
+	if strings.Contains(b.Path, "daily") {
+		start = end.AddDate(0, 0, -(period - 1))
+		path = strings.Replace(b.Path, "{start}", start.Format("2006-01-02"), 1)
+		path = strings.Replace(path, "{end}", end.Format("2006-01-02"), 1)
+	} else {
+		path = strings.Replace(b.Path, "{end}", end.Format("2006-01-02"), 1)
+		path = strings.Replace(path, "{period}", fmt.Sprintf("%d", period), 1)
+	}
+
+	data, err := client.ConnectAPI(path, "GET", nil, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(data) == 0 {
+		return []interface{}{}, nil
+	}
+
+	var responseSlice []map[string]interface{}
+	if err := json.Unmarshal(data, &responseSlice); err != nil {
+		return nil, err
+	}
+
+	if len(responseSlice) == 0 {
+		return []interface{}{}, nil
+	}
+
+	var results []interface{}
+	for _, itemMap := range responseSlice {
+		// Handle nested "values" structure
+		if values, exists := itemMap["values"]; exists {
+			valuesMap := values.(map[string]interface{})
+			for k, v := range valuesMap {
+				itemMap[k] = v
+			}
+			delete(itemMap, "values")
+		}
+
+		snakeItem := utils.CamelToSnakeDict(itemMap)
+		results = append(results, snakeItem)
+	}
+
+	return results, nil
+}