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.

pkg/garmin/health.go

@@ -0,0 +1,58 @@
+package garmin
+
+import (
+	"garmin-connect/internal/data"
+	"time"
+)
+
+// BodyBatteryData represents Body Battery data.
+type BodyBatteryData = data.DailyBodyBatteryStress
+
+// SleepData represents sleep data.
+type SleepData = data.DailySleepDTO
+
+// HRVData represents HRV data.
+type HRVData = data.HRVData
+
+// WeightData represents weight data.
+type WeightData = data.WeightData
+
+// GetBodyBattery retrieves Body Battery data for a given date.
+func (c *Client) GetBodyBattery(date time.Time) (*BodyBatteryData, error) {
+	bb := &data.DailyBodyBatteryStress{}
+	result, err := bb.Get(date, c.Client)
+	if err != nil {
+		return nil, err
+	}
+	return result.(*BodyBatteryData), nil
+}
+
+// GetSleep retrieves sleep data for a given date.
+func (c *Client) GetSleep(date time.Time) (*SleepData, error) {
+	sleep := &data.DailySleepDTO{}
+	result, err := sleep.Get(date, c.Client)
+	if err != nil {
+		return nil, err
+	}
+	return result.(*SleepData), nil
+}
+
+// GetHRV retrieves HRV data for a given date.
+func (c *Client) GetHRV(date time.Time) (*HRVData, error) {
+	hrv := &data.HRVData{}
+	result, err := hrv.Get(date, c.Client)
+	if err != nil {
+		return nil, err
+	}
+	return result.(*HRVData), nil
+}
+
+// GetWeight retrieves weight data for a given date.
+func (c *Client) GetWeight(date time.Time) (*WeightData, error) {
+	weight := &data.WeightData{}
+	result, err := weight.Get(date, c.Client)
+	if err != nil {
+		return nil, err
+	}
+	return result.(*WeightData), nil
+}