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/auth_models.go

@@ -112,3 +112,8 @@ type PasswordChangeRequest struct {
 	CurrentPassword string `json:"currentPassword"`
 	NewPassword     string `json:"newPassword"`
 }
+
+type CheckLoginIDRequest struct {
+	LoginID     string `json:"loginId"`
+	CompanyCode string `json:"companyCode,omitempty"`
+}