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/body_battery.go

@@ -0,0 +1,138 @@
+package data
+
+import (
+	"encoding/json"
+	"fmt"
+	"sort"
+	"time"
+
+	"garmin-connect/internal/api/client"
+)
+
+// DailyBodyBatteryStress represents complete daily Body Battery and stress data
+type DailyBodyBatteryStress struct {
+	UserProfilePK          int       `json:"userProfilePk"`
+	CalendarDate           time.Time `json:"calendarDate"`
+	StartTimestampGMT      time.Time `json:"startTimestampGmt"`
+	EndTimestampGMT        time.Time `json:"endTimestampGmt"`
+	StartTimestampLocal    time.Time `json:"startTimestampLocal"`
+	EndTimestampLocal      time.Time `json:"endTimestampLocal"`
+	MaxStressLevel         int       `json:"maxStressLevel"`
+	AvgStressLevel         int       `json:"avgStressLevel"`
+	StressChartValueOffset int       `json:"stressChartValueOffset"`
+	StressChartYAxisOrigin int       `json:"stressChartYAxisOrigin"`
+	StressValuesArray      [][]int   `json:"stressValuesArray"`
+	BodyBatteryValuesArray [][]any   `json:"bodyBatteryValuesArray"`
+}
+
+// BodyBatteryEvent represents a Body Battery impact event
+type BodyBatteryEvent struct {
+	EventType              string    `json:"eventType"`
+	EventStartTimeGMT      time.Time `json:"eventStartTimeGmt"`
+	TimezoneOffset         int       `json:"timezoneOffset"`
+	DurationInMilliseconds int       `json:"durationInMilliseconds"`
+	BodyBatteryImpact      int       `json:"bodyBatteryImpact"`
+	FeedbackType           string    `json:"feedbackType"`
+	ShortFeedback          string    `json:"shortFeedback"`
+}
+
+// BodyBatteryData represents legacy Body Battery events data
+type BodyBatteryData struct {
+	Event                  *BodyBatteryEvent `json:"event"`
+	ActivityName           string            `json:"activityName"`
+	ActivityType           string            `json:"activityType"`
+	ActivityID             string            `json:"activityId"`
+	AverageStress          float64           `json:"averageStress"`
+	StressValuesArray      [][]int           `json:"stressValuesArray"`
+	BodyBatteryValuesArray [][]any           `json:"bodyBatteryValuesArray"`
+}
+
+// BodyBatteryReading represents an individual Body Battery reading
+type BodyBatteryReading struct {
+	Timestamp int     `json:"timestamp"`
+	Status    string  `json:"status"`
+	Level     int     `json:"level"`
+	Version   float64 `json:"version"`
+}
+
+// StressReading represents an individual stress reading
+type StressReading struct {
+	Timestamp   int `json:"timestamp"`
+	StressLevel int `json:"stressLevel"`
+}
+
+// ParseBodyBatteryReadings converts body battery values array to structured readings
+func ParseBodyBatteryReadings(valuesArray [][]any) []BodyBatteryReading {
+	readings := make([]BodyBatteryReading, 0)
+	for _, values := range valuesArray {
+		if len(values) < 4 {
+			continue
+		}
+
+		timestamp, ok1 := values[0].(int)
+		status, ok2 := values[1].(string)
+		level, ok3 := values[2].(int)
+		version, ok4 := values[3].(float64)
+
+		if !ok1 || !ok2 || !ok3 || !ok4 {
+			continue
+		}
+
+		readings = append(readings, BodyBatteryReading{
+			Timestamp: timestamp,
+			Status:    status,
+			Level:     level,
+			Version:   version,
+		})
+	}
+	sort.Slice(readings, func(i, j int) bool {
+		return readings[i].Timestamp < readings[j].Timestamp
+	})
+	return readings
+}
+
+// ParseStressReadings converts stress values array to structured readings
+func ParseStressReadings(valuesArray [][]int) []StressReading {
+	readings := make([]StressReading, 0)
+	for _, values := range valuesArray {
+		if len(values) != 2 {
+			continue
+		}
+		readings = append(readings, StressReading{
+			Timestamp:   values[0],
+			StressLevel: values[1],
+		})
+	}
+	sort.Slice(readings, func(i, j int) bool {
+		return readings[i].Timestamp < readings[j].Timestamp
+	})
+	return readings
+}
+
+// Get implements the Data interface for DailyBodyBatteryStress
+func (d *DailyBodyBatteryStress) Get(day time.Time, client *client.Client) (any, error) {
+	dateStr := day.Format("2006-01-02")
+	path := fmt.Sprintf("/wellness-service/wellness/dailyStress/%s", dateStr)
+
+	data, err := client.ConnectAPI(path, "GET", nil, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(data) == 0 {
+		return nil, nil
+	}
+
+	var result DailyBodyBatteryStress
+	if err := json.Unmarshal(data, &result); err != nil {
+		return nil, err
+	}
+
+	return &result, nil
+}
+
+// List implements the Data interface for concurrent fetching
+func (d *DailyBodyBatteryStress) List(end time.Time, days int, client *client.Client, maxWorkers int) ([]any, error) {
+	// Implementation to be added
+	return []any{}, nil
+}