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.

userfront/lib/core/services/auth_proxy_service.dart

@@ -893,6 +893,27 @@ class AuthProxyService {
     return false;
   }
 
+  static Future<Map<String, dynamic>> checkLoginIDAvailability(String loginId, {String? companyCode}) async {
+    final url = Uri.parse('$_baseUrl/api/v1/auth/signup/check-login-id');
+    final bodyData = {'loginId': loginId};
+    if (companyCode != null && companyCode.isNotEmpty) {
+      bodyData['companyCode'] = companyCode;
+    }
+    final response = await http.post(
+      url,
+      headers: {'Content-Type': 'application/json'},
+      body: jsonEncode(bodyData),
+    );
+
+    if (response.statusCode == 200) {
+      final data = jsonDecode(response.body);
+      return {'available': data['available'] ?? false, 'message': data['message']};
+    } else {
+      final data = jsonDecode(response.body);
+      return {'available': false, 'message': data['message'] ?? 'Failed to check ID'};
+    }
+  }
+
   static Future<void> sendSignupCode(String target, String type) async {
     final path = type == 'email' ? 'send-email-code' : 'send-sms-code';
     final url = Uri.parse('$_baseUrl/api/v1/auth/signup/$path');

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

@@ -59,6 +59,8 @@ class _SignupScreenState extends State<SignupScreen> {
   String? _phoneError;
   String? _passwordError;
   String? _confirmPasswordError;
+  String? _loginIdError;
+  String? _loginIdSuccess;
 
   // Timers
   Timer? _emailTimer;
@@ -1428,12 +1430,61 @@ class _SignupScreenState extends State<SignupScreen> {
                       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(),
-                        ),
+                      child: Column(
+                        crossAxisAlignment: CrossAxisAlignment.stretch,
+                        children: [
+                          TextFormField(
+                            controller: _loginIdController,
+                            onChanged: (val) {
+                              setState(() {
+                                _loginIdError = null;
+                                _loginIdSuccess = null;
+                              });
+                            },
+                            decoration: InputDecoration(
+                              labelText: '사번 또는 아이디',
+                              border: const OutlineInputBorder(),
+                              errorText: _loginIdError,
+                              suffixIcon: TextButton(
+                                onPressed: _isLoading ? null : () async {
+                                  final loginId = _loginIdController.text.trim();
+                                  if (loginId.isEmpty) {
+                                    setState(() => _loginIdError = 'ID를 입력해주세요.');
+                                    return;
+                                  }
+                                  setState(() {
+                                    _isLoading = true;
+                                    _loginIdError = null;
+                                    _loginIdSuccess = null;
+                                  });
+                                  try {
+                                    final result = await AuthProxyService.checkLoginIDAvailability(loginId, companyCode: _affiliationType == 'AFFILIATE' ? _companyCode : null);
+                                    setState(() {
+                                      if (result['available'] == true) {
+                                        _loginIdSuccess = '사용 가능한 ID입니다.';
+                                      } else {
+                                        _loginIdError = result['message'] ?? '사용할 수 없는 ID입니다.';
+                                      }
+                                    });
+                                  } catch (e) {
+                                    setState(() => _loginIdError = e.toString().replaceAll('Exception: ', ''));
+                                  } finally {
+                                    if (mounted) setState(() => _isLoading = false);
+                                  }
+                                },
+                                child: const Text('중복 확인'),
+                              ),
+                            ),
+                          ),
+                          if (_loginIdSuccess != null)
+                            Padding(
+                              padding: const EdgeInsets.only(top: 8.0, left: 12.0),
+                              child: Text(
+                                _loginIdSuccess!,
+                                style: const TextStyle(color: Colors.green, fontSize: 12),
+                              ),
+                            ),
+                        ],
                       ),
                     ),
                     const SizedBox(height: 18),