feat: création services singleton et renommage Box

Services créés:
✅ CurrentUserService singleton pour utilisateur connecté
✅ CurrentAmicaleService singleton pour amicale courante
✅ ApiService transformé en singleton

Box Hive:
✅ Renommage users -> user (plus logique)
✅ Migration automatique des données
✅ Services intégrés dans main.dart

État: Services créés, prêt pour refactorisation repositories

app/.dart_tool/flutter_build/41acb28aedc1da36af63ba5cb8859018/app.dill.deps

@@ -1775,7 +1775,6 @@ file:///Users/pierre/dev/geosector/app/lib/core/repositories/client_repository.d
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/membre_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/operation_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/passage_repository.dart
-file:///Users/pierre/dev/geosector/app/lib/core/repositories/region_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/sector_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/user_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/services/api_service.dart

app/.dart_tool/flutter_build/41acb28aedc1da36af63ba5cb8859018/main.dart.js.deps

@@ -1774,7 +1774,6 @@ file:///Users/pierre/dev/geosector/app/lib/core/repositories/client_repository.d
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/membre_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/operation_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/passage_repository.dart
-file:///Users/pierre/dev/geosector/app/lib/core/repositories/region_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/sector_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/repositories/user_repository.dart
 file:///Users/pierre/dev/geosector/app/lib/core/services/api_service.dart