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/weight.go

@@ -0,0 +1,81 @@
+package data
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"garmin-connect/internal/api/client"
+)
+
+// WeightData represents weight measurement data
+type WeightData struct {
+	UserProfilePK     int       `json:"userProfilePk"`
+	CalendarDate      time.Time `json:"calendarDate"`
+	Timestamp         time.Time `json:"timestamp"`
+	Weight            float64   `json:"weight"` // in kilograms
+	BMI               float64   `json:"bmi"`
+	BodyFatPercentage float64   `json:"bodyFatPercentage"`
+	BoneMass          float64   `json:"boneMass"`   // in kg
+	MuscleMass        float64   `json:"muscleMass"` // in kg
+	Hydration         float64   `json:"hydration"`  // in kg
+	BaseData
+}
+
+// Validate checks if weight data contains valid values
+func (w *WeightData) Validate() error {
+	if w.Weight <= 0 {
+		return fmt.Errorf("invalid weight value")
+	}
+	if w.BMI < 10 || w.BMI > 50 {
+		return fmt.Errorf("BMI out of valid range")
+	}
+	return nil
+}
+
+// Get implements the Data interface for WeightData
+func (w *WeightData) Get(day time.Time, client *client.Client) (any, error) {
+	startDate := day.Format("2006-01-02")
+	endDate := day.Format("2006-01-02")
+	path := fmt.Sprintf("/weight-service/weight/dateRange?startDate=%s&endDate=%s",
+		startDate, endDate)
+
+	data, err := client.ConnectAPI(path, "GET", nil, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(data) == 0 {
+		return nil, nil
+	}
+
+	var response struct {
+		WeightList []WeightData `json:"weightList"`
+	}
+	if err := json.Unmarshal(data, &response); err != nil {
+		return nil, err
+	}
+
+	if len(response.WeightList) == 0 {
+		return nil, nil
+	}
+
+	weightData := response.WeightList[0]
+	// Convert grams to kilograms
+	weightData.Weight = weightData.Weight / 1000
+	weightData.BoneMass = weightData.BoneMass / 1000
+	weightData.MuscleMass = weightData.MuscleMass / 1000
+	weightData.Hydration = weightData.Hydration / 1000
+
+	return weightData, nil
+}
+
+// List implements the Data interface for concurrent fetching
+func (w *WeightData) List(end time.Time, days int, client *client.Client, maxWorkers int) ([]any, error) {
+	results, errs := w.BaseData.List(end, days, client, maxWorkers)
+	if len(errs) > 0 {
+		// Return first error for now
+		return results, errs[0]
+	}
+	return results, nil
+}