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,