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/hydration.go

@@ -0,0 +1,20 @@
+package stats
+
+import "time"
+
+const BASE_HYDRATION_PATH = "/usersummary-service/stats/hydration"
+
+type DailyHydration struct {
+	CalendarDate time.Time `json:"calendar_date"`
+	TotalWaterML *int      `json:"total_water_ml"`
+	BaseStats
+}
+
+func NewDailyHydration() *DailyHydration {
+	return &DailyHydration{
+		BaseStats: BaseStats{
+			Path:     BASE_HYDRATION_PATH + "/daily/{start}/{end}",
+			PageSize: 28,
+		},
+	}
+}