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/count/count_state.dart

@@ -0,0 +1,42 @@
+import 'package:equatable/equatable.dart';
+import '../../data/models/expense.dart';
+import '../../data/models/balance.dart';
+
+abstract class CountState extends Equatable {
+  const CountState();
+
+  @override
+  List<Object?> get props => [];
+}
+
+class CountInitial extends CountState {}
+
+class CountLoading extends CountState {}
+
+class ExpensesLoaded extends CountState {
+  final String groupId;
+  final List<Expense> expenses;
+  final List<Balance> balances;
+  final List<Settlement> settlements;
+  final Map<ExpenseCurrency, double> exchangeRates;
+
+  const ExpensesLoaded({
+    required this.groupId,
+    required this.expenses,
+    required this.balances,
+    required this.settlements,
+    required this.exchangeRates,
+  });
+
+  @override
+  List<Object?> get props => [groupId, expenses, balances, settlements, exchangeRates];
+}
+
+class CountError extends CountState {
+  final String message;
+
+  const CountError(this.message);
+
+  @override
+  List<Object?> get props => [message];
+}