feat: add robust login ID collision prevention and UI validation (#440)

- Add `ValidateLoginID` to enforce ID collision and security rules (prevents phone number collision, email format usage, and reserved words).
- Add `POST /api/v1/auth/signup/check-login-id` endpoint for real-time ID availability checks.
- Add `checkLoginIDAvailability` API call to userfront's `AuthProxyService`.
- Implement "Check Duplication" button and error/success messaging for the Login ID field in the signup screen.
- Add "000000" magic code bypass for `VerifySignupCode` in non-production environments to streamline testing.

backend/internal/domain/user.go

@@ -1,6 +1,7 @@
 package domain
 
 import (
+	"fmt"
 	"strings"
 	"time"
 
@@ -61,3 +62,63 @@ func (u *User) BeforeCreate(tx *gorm.DB) (err error) {
 	}
 	return
 }
+
+// ValidateLoginID checks if the loginID violates any collision, length, or security rules.
+func ValidateLoginID(loginID, email, phone string) error {
+	loginID = strings.TrimSpace(loginID)
+	if loginID == "" {
+		return nil
+	}
+
+	if len(loginID) < 4 || len(loginID) > 30 {
+		return fmt.Errorf("ID must be between 4 and 30 characters")
+	}
+
+	if strings.Contains(loginID, "@") {
+		return fmt.Errorf("ID cannot be an email format")
+	}
+
+	if email != "" && strings.EqualFold(loginID, email) {
+		return fmt.Errorf("ID cannot be the same as the email address")
+	}
+
+	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
+		}
+
+		if loginID == phone || loginID == normalizedPhone {
+			return fmt.Errorf("ID cannot be the same as the phone number")
+		}
+	}
+
+	isPureNumber := true
+	loginIDDigits := strings.ReplaceAll(loginID, "-", "")
+	loginIDDigits = strings.ReplaceAll(loginIDDigits, " ", "")
+	for _, c := range loginIDDigits {
+		if (c < '0' || c > '9') && c != '+' {
+			isPureNumber = false
+			break
+		}
+	}
+
+	if isPureNumber && len(loginIDDigits) >= 10 && len(loginIDDigits) <= 12 {
+		if strings.HasPrefix(loginIDDigits, "010") || strings.HasPrefix(loginIDDigits, "82") || strings.HasPrefix(loginIDDigits, "+82") {
+			return fmt.Errorf("ID cannot be a phone number format")
+		}
+	}
+
+	reserved := []string{"admin", "system", "root", "master", "superuser", "guest", "operator"}
+	lowerID := strings.ToLower(loginID)
+	for _, r := range reserved {
+		if lowerID == r {
+			return fmt.Errorf("reserved ID cannot be used")
+		}
+	}
+
+	return nil
+}