feat: Add library directive to multiple BLoC and service files for improved organization

lib/services/balance_service.dart

@@ -31,6 +31,7 @@
 /// - [GroupBalance] for the complete balance structure
 /// - [Settlement] for individual payment recommendations
 /// - [UserBalance] for per-user balance information
+library;
 import '../models/group_balance.dart';
 import '../models/expense.dart';
 import '../models/group_statistics.dart';