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/hrv_weekly.go

@@ -0,0 +1,40 @@
+package stats
+
+import (
+	"errors"
+	"time"
+)
+
+const WEEKLY_HRV_PATH = "/wellness-service/wellness/weeklyHrv"
+
+type WeeklyHRV struct {
+	CalendarDate          time.Time `json:"calendar_date"`
+	AverageHRV            float64   `json:"average_hrv"`
+	MaxHRV                float64   `json:"max_hrv"`
+	MinHRV                float64   `json:"min_hrv"`
+	HRVQualifier          string    `json:"hrv_qualifier"`
+	WellnessDataDaysCount int       `json:"wellness_data_days_count"`
+	BaseStats
+}
+
+func NewWeeklyHRV() *WeeklyHRV {
+	return &WeeklyHRV{
+		BaseStats: BaseStats{
+			Path:     WEEKLY_HRV_PATH + "/{end}/{period}",
+			PageSize: 52,
+		},
+	}
+}
+
+func (w *WeeklyHRV) Validate() error {
+	if w.CalendarDate.IsZero() {
+		return errors.New("calendar_date is required")
+	}
+	if w.AverageHRV < 0 || w.MaxHRV < 0 || w.MinHRV < 0 {
+		return errors.New("HRV values must be non-negative")
+	}
+	if w.MaxHRV < w.MinHRV {
+		return errors.New("max_hrv must be greater than min_hrv")
+	}
+	return nil
+}