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';

lib/services/storage_service.dart

@@ -24,6 +24,7 @@
 /// // Delete a file
 /// await storageService.deleteFile(fileUrl);
 /// ```
+library;
 import 'dart:io';
 import 'dart:typed_data';
 import 'package:firebase_storage/firebase_storage.dart';

lib/services/trip_service.dart

@@ -24,6 +24,7 @@
 /// // Calculate trip statistics
 /// final stats = await tripService.getTripStatistics(tripId);
 /// ```
+library;
 import 'package:cloud_firestore/cloud_firestore.dart';
 import 'package:travel_mate/services/error_service.dart';
 import '../models/trip.dart';