chore: consolidate local integration changes

backend/internal/domain/user.go

@@ -174,13 +174,7 @@ func ValidateLoginID(loginID string, emails []string, phone string) error {
 	}
 
 	if phone != "" {
-		normalizedPhone := strings.ReplaceAll(phone, "-", "")
-		normalizedPhone = strings.ReplaceAll(normalizedPhone, " ", "")
-		if strings.HasPrefix(normalizedPhone, "010") {
-			normalizedPhone = "+82" + normalizedPhone[1:]
-		} else if strings.HasPrefix(normalizedPhone, "82") {
-			normalizedPhone = "+" + normalizedPhone
-		}
+		normalizedPhone := NormalizePhoneNumber(phone)
 
 		if loginID == phone || loginID == normalizedPhone {
 			return fmt.Errorf("ID cannot be the same as the phone number")
@@ -211,3 +205,43 @@ func ValidateLoginID(loginID string, emails []string, phone string) error {
 
 	return nil
 }
+
+func NormalizePhoneNumber(phone string) string {
+	trimmed := strings.TrimSpace(phone)
+	if trimmed == "" {
+		return ""
+	}
+
+	hasLeadingPlus := false
+	digits := strings.Builder{}
+	for _, r := range trimmed {
+		switch {
+		case r >= '0' && r <= '9':
+			digits.WriteRune(r)
+		case r == '+' && digits.Len() == 0 && !hasLeadingPlus:
+			hasLeadingPlus = true
+		}
+	}
+
+	number := digits.String()
+	if number == "" {
+		return ""
+	}
+	if strings.HasPrefix(number, "010") {
+		return "+82" + number[1:]
+	}
+	if strings.HasPrefix(number, "82") {
+		rest := number[2:]
+		for strings.HasPrefix(rest, "82") {
+			rest = rest[2:]
+		}
+		if strings.HasPrefix(rest, "0") {
+			rest = rest[1:]
+		}
+		return "+82" + rest
+	}
+	if hasLeadingPlus {
+		return "+" + number
+	}
+	return number
+}