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_event.dart

@@ -0,0 +1,91 @@
+import 'dart:io';
+import 'package:equatable/equatable.dart';
+import '../../data/models/expense.dart';
+
+abstract class CountEvent extends Equatable {
+  const CountEvent();
+
+  @override
+  List<Object?> get props => [];
+}
+
+class LoadExpenses extends CountEvent {
+  final String groupId;
+  final bool includeArchived;
+
+  const LoadExpenses(this.groupId, {this.includeArchived = false});
+
+  @override
+  List<Object?> get props => [groupId, includeArchived];
+}
+
+class CreateExpense extends CountEvent {
+  final Expense expense;
+  final File? receiptImage;
+
+  const CreateExpense({
+    required this.expense,
+    this.receiptImage,
+  });
+
+  @override
+  List<Object?> get props => [expense, receiptImage];
+}
+
+class UpdateExpense extends CountEvent {
+  final Expense expense;
+  final File? newReceiptImage;
+
+  const UpdateExpense({
+    required this.expense,
+    this.newReceiptImage,
+  });
+
+  @override
+  List<Object?> get props => [expense, newReceiptImage];
+}
+
+class DeleteExpense extends CountEvent {
+  final String groupId;
+  final String expenseId;
+
+  const DeleteExpense({
+    required this.groupId,
+    required this.expenseId,
+  });
+
+  @override
+  List<Object?> get props => [groupId, expenseId];
+}
+
+class ArchiveExpense extends CountEvent {
+  final String groupId;
+  final String expenseId;
+
+  const ArchiveExpense({
+    required this.groupId,
+    required this.expenseId,
+  });
+
+  @override
+  List<Object?> get props => [groupId, expenseId];
+}
+
+class MarkSplitAsPaid extends CountEvent {
+  final String groupId;
+  final String expenseId;
+  final String userId;
+
+  const MarkSplitAsPaid({
+    required this.groupId,
+    required this.expenseId,
+    required this.userId,
+  });
+
+  @override
+  List<Object?> get props => [groupId, expenseId, userId];
+}
+
+class LoadExchangeRates extends CountEvent {
+  const LoadExchangeRates();
+}