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/stats.go

@@ -0,0 +1,38 @@
+package garmin
+
+import (
+	"garmin-connect/internal/stats"
+)
+
+// Stats is an interface for stats data types.
+type Stats = stats.Stats
+
+// NewDailySteps creates a new DailySteps stats type.
+func NewDailySteps() Stats {
+	return stats.NewDailySteps()
+}
+
+// NewDailyStress creates a new DailyStress stats type.
+func NewDailyStress() Stats {
+	return stats.NewDailyStress()
+}
+
+// NewDailyHydration creates a new DailyHydration stats type.
+func NewDailyHydration() Stats {
+	return stats.NewDailyHydration()
+}
+
+// NewDailyIntensityMinutes creates a new DailyIntensityMinutes stats type.
+func NewDailyIntensityMinutes() Stats {
+	return stats.NewDailyIntensityMinutes()
+}
+
+// NewDailySleep creates a new DailySleep stats type.
+func NewDailySleep() Stats {
+	return stats.NewDailySleep()
+}
+
+// NewDailyHRV creates a new DailyHRV stats type.
+func NewDailyHRV() Stats {
+	return stats.NewDailyHRV()
+}