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

lib/blocs/group/group_state.dart

@@ -24,6 +24,12 @@ class GroupsLoaded extends GroupState {
   List<Object?> get props => [groups];
 }
 
+class GroupLoaded extends GroupState {
+  final List<Group> groups;
+
+  const GroupLoaded(this.groups);
+}
+
 // Succès d'une opération
 class GroupOperationSuccess extends GroupState {
   final String message;