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

userfront/lib/core/services/auth_proxy_service.dart

@@ -923,13 +923,19 @@ class AuthProxyService {
     }
   }
 
-  static Future<List<Map<String, dynamic>>> getActiveTenants() async {
-    final url = Uri.parse('$_baseUrl/api/v1/auth/signup/tenants');
-    final response = await http.get(url);
+  static Future<List<Map<String, dynamic>>> getActiveTenants({
+    String? email,
+  }) async {
+    var uriString = '$_baseUrl/api/v1/auth/signup/tenants';
+    if (email != null && email.isNotEmpty) {
+      uriString += '?email=${Uri.encodeComponent(email)}';
+    }
 
+    final url = Uri.parse(uriString);
+    final response = await http.get(url);
     if (response.statusCode == 200) {
-      final List<dynamic> data = jsonDecode(response.body);
-      return data.cast<Map<String, dynamic>>();
+      final List<dynamic> list = jsonDecode(response.body);
+      return list.cast<Map<String, dynamic>>();
     }
     return [];
   }
@@ -953,7 +959,7 @@ class AuthProxyService {
     }
   }
 
-  static Future<bool> verifySignupCode(
+  static Future<Map<String, dynamic>> verifySignupCode(
     String target,
     String type,
     String code,
@@ -967,10 +973,9 @@ class AuthProxyService {
     );
 
     if (response.statusCode == 200) {
-      final data = jsonDecode(response.body);
-      return data['success'] ?? false;
+      return jsonDecode(response.body);
     }
-    return false;
+    throw Exception('Verification failed');
   }
 
   static Future<void> signup({