feat: restore explicit loginId field and add to userfront signup flow

- Revert the removal of loginId from adminfront and backend.
- Prevent phone normalization logic from mangling custom employee ID login fields.
- Add an explicit 'loginId' optional input field to the userfront signup UI.
- Update AuthProxyService.signup and backend AuthHandler.Signup to transmit and map the 'loginId' parameter properly.

userfront/lib/core/services/auth_proxy_service.dart

@@ -934,6 +934,7 @@ class AuthProxyService {
 
   static Future<void> signup({
     required String email,
+    String? loginId,
     required String password,
     required String name,
     required String phone,
@@ -949,6 +950,7 @@ class AuthProxyService {
       headers: {'Content-Type': 'application/json'},
       body: jsonEncode({
         'email': email,
+        if (loginId != null && loginId.isNotEmpty) 'loginId': loginId,
         'password': password,
         'name': name,
         'phone': phone,

userfront/lib/features/auth/presentation/signup_screen.dart

@@ -31,6 +31,7 @@ class _SignupScreenState extends State<SignupScreen> {
 
   // Controllers
   final _emailController = TextEditingController();
+  final _loginIdController = TextEditingController();
   final _emailCodeController = TextEditingController();
   final _phoneController = TextEditingController();
   final _phoneCodeController = TextEditingController();
@@ -98,6 +99,7 @@ class _SignupScreenState extends State<SignupScreen> {
     _emailTimer?.cancel();
     _phoneTimer?.cancel();
     _emailController.dispose();
+    _loginIdController.dispose();
     _emailCodeController.dispose();
     _phoneController.dispose();
     _phoneCodeController.dispose();
@@ -311,6 +313,7 @@ class _SignupScreenState extends State<SignupScreen> {
     try {
       await AuthProxyService.signup(
         email: _emailController.text.trim(),
+        loginId: _loginIdController.text.trim(),
         password: _passwordController.text,
         name: _nameController.text.trim(),
         phone: _phoneController.text.trim(),
@@ -1421,6 +1424,19 @@ class _SignupScreenState extends State<SignupScreen> {
                       ),
                     ),
                     const SizedBox(height: 18),
+                    _buildProfileFieldGroup(
+                      title: '로그인 ID (선택)',
+                      description: '이메일/전화번호 외에 별도의 식별자로 로그인할 때 사용합니다.',
+                      isDesktop: isDesktop,
+                      child: TextFormField(
+                        controller: _loginIdController,
+                        decoration: const InputDecoration(
+                          labelText: '사번 또는 아이디',
+                          border: OutlineInputBorder(),
+                        ),
+                      ),
+                    ),
+                    const SizedBox(height: 18),
                     _buildProfileFieldGroup(
                       title: tr('ui.userfront.signup.profile.affiliation_type'),
                       description: _isAffiliateEmail