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/utils/timeutils.go

@@ -0,0 +1,38 @@
+package utils
+
+import (
+	"time"
+)
+
+var (
+	// Default location for conversions (set to UTC by default)
+	defaultLocation *time.Location
+)
+
+func init() {
+	var err error
+	defaultLocation, err = time.LoadLocation("UTC")
+	if err != nil {
+		panic(err)
+	}
+}
+
+// SetDefaultLocation sets the default time location for conversions
+func SetDefaultLocation(loc *time.Location) {
+	defaultLocation = loc
+}
+
+// ParseTimestamp converts a millisecond timestamp to time.Time in default location
+func ParseTimestamp(ts int) time.Time {
+	return time.Unix(0, int64(ts)*int64(time.Millisecond)).In(defaultLocation)
+}
+
+// ToLocalTime converts UTC time to local time using default location
+func ToLocalTime(utcTime time.Time) time.Time {
+	return utcTime.In(defaultLocation)
+}
+
+// ToUTCTime converts local time to UTC
+func ToUTCTime(localTime time.Time) time.Time {
+	return localTime.UTC()
+}