feat: Add expense management features with tabs for expenses, balances, and settlements

- Implemented ExpensesTab to display a list of expenses with details.
- Created GroupExpensesPage to manage group expenses with a tabbed interface.
- Added SettlementsTab to show optimized settlements between users.
- Developed data models for Expense and Balance, including necessary methods for serialization.
- Introduced CountRepository for Firestore interactions related to expenses.
- Added CountService to handle business logic for expenses and settlements.
- Integrated image picker for receipt uploads.
- Updated main.dart to include CountBloc and CountRepository.
- Enhanced pubspec.yaml with new dependencies for image picking and Firebase storage.

Not Tested yet

pubspec.yaml

@@ -51,6 +51,9 @@ dependencies:
   google_places_flutter: ^2.1.1
   http: ^1.5.0
   flutter_dotenv: ^6.0.0
+  image_picker: ^1.2.0
+  intl: ^0.20.2
+  firebase_storage: ^13.0.3
 
 dev_dependencies:
   flutter_launcher_icons: ^0.13.1