feat(auth): lock affiliation type on frontend based on verified email domain (#500)

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

@@ -44,6 +44,7 @@ class _SignupScreenState extends State<SignupScreen> {
   bool _isEmailVerified = false;
   bool _isPhoneVerified = false;
   String _affiliationType = 'GENERAL';
+  bool _isAffiliateLocked = false;
   String? _companyCode;
   bool _termsAccepted = false;
   bool _privacyAccepted = false;
@@ -72,15 +73,23 @@ class _SignupScreenState extends State<SignupScreen> {
   void initState() {
     super.initState();
     _loadPolicy();
-    _fetchTenants();
+    // initState에서는 _fetchTenants() 호출 제외
   }
 
   Future<void> _fetchTenants() async {
+    if (!_isEmailVerified) return;
+
     try {
-      final tenants = await AuthProxyService.getActiveTenants();
+      final tenants = await AuthProxyService.getActiveTenants(
+        email: _emailController.text.trim(),
+      );
       if (mounted) {
         setState(() {
           _tenants = tenants;
+          if (_tenants.isNotEmpty && _affiliationType == 'AFFILIATE') {
+            // 목록이 있는데 아직 아무것도 선택되지 않았다면 자동 할당 가능
+            _companyCode ??= _tenants.first['slug'];
+          }
         });
       }
     } catch (e) {
@@ -195,18 +204,32 @@ class _SignupScreenState extends State<SignupScreen> {
     final code = _emailCodeController.text.trim();
     if (code.length != 6) return;
     try {
-      final success = await AuthProxyService.verifySignupCode(
+      final res = await AuthProxyService.verifySignupCode(
         _emailController.text.trim(),
         'email',
         code,
       );
-      if (success) {
+      if (res['success'] == true) {
         setState(() {
           _isEmailVerified = true;
           _emailTimer?.cancel();
           _emailSeconds = 0;
           _emailError = null;
+
+          if (res['isAffiliate'] == true) {
+            _affiliationType = 'AFFILIATE';
+            _isAffiliateLocked = true;
+          } else {
+            _affiliationType = 'GENERAL';
+            _companyCode = null;
+            _isAffiliateLocked = true;
+          }
         });
+        
+        // Only fetch tenants if it's an affiliate domain
+        if (res['isAffiliate'] == true) {
+          _fetchTenants();
+        }
       } else {
         setState(
           () => _emailError = tr('msg.userfront.signup.email.code_mismatch'),
@@ -248,12 +271,12 @@ class _SignupScreenState extends State<SignupScreen> {
     final code = _phoneCodeController.text.trim();
     if (code.length != 6) return;
     try {
-      final success = await AuthProxyService.verifySignupCode(
+      final res = await AuthProxyService.verifySignupCode(
         _phoneController.text.trim(),
         'phone',
         code,
       );
-      if (success) {
+      if (res['success'] == true) {
         setState(() {
           _isPhoneVerified = true;
           _phoneTimer?.cancel();
@@ -1445,17 +1468,19 @@ class _SignupScreenState extends State<SignupScreen> {
                                 ),
                               ),
                             ],
-                            onChanged: (val) {
-                              if (val == null) {
-                                return;
-                              }
-                              setState(() {
-                                _affiliationType = val;
-                                if (_affiliationType == 'GENERAL') {
-                                  _companyCode = null;
-                                }
-                              });
-                            },
+                            onChanged: _isAffiliateLocked 
+                                ? null 
+                                : (val) {
+                                    if (val == null) {
+                                      return;
+                                    }
+                                    setState(() {
+                                      _affiliationType = val;
+                                      if (_affiliationType == 'GENERAL') {
+                                        _companyCode = null;
+                                      }
+                                    });
+                                  },
                           ),
                           AnimatedSize(
                             duration: const Duration(milliseconds: 180),