Change registerServer method to return operational seed after successful registration

lib/services/server_service.dart

@@ -6,7 +6,7 @@ import 'package:waylume_server/core/utils.dart';
 import 'package:waylume_server/services/rolling_codes_service.dart';
 
 class ServerService {
-  static Future<void> registerServer() async {
+  static Future<String> registerServer() async {
     await RollingCodesService.initialize();
     
     // Server always registers on startup to get fresh operational seed
@@ -60,6 +60,7 @@ class ServerService {
           // Store operational seed in memory (no persistence needed)
           RollingCodesService.setOperationalSeed(responseData['operational_seed']);
           print('Server registered successfully with server-manager');
+          return responseData['operational_seed'];
         } else {
           throw Exception('Registration failed: ${responseData['error']}');
         }