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/data/models/balance.dart

@@ -0,0 +1,34 @@
+class Balance {
+  final String userId;
+  final String userName;
+  final double totalPaid;
+  final double totalOwed;
+  final double balance;
+
+  Balance({
+    required this.userId,
+    required this.userName,
+    required this.totalPaid,
+    required this.totalOwed,
+  }) : balance = totalPaid - totalOwed;
+
+  bool get shouldReceive => balance > 0;
+  bool get shouldPay => balance < 0;
+  double get absoluteBalance => balance.abs();
+}
+
+class Settlement {
+  final String fromUserId;
+  final String fromUserName;
+  final String toUserId;
+  final String toUserName;
+  final double amount;
+
+  Settlement({
+    required this.fromUserId,
+    required this.fromUserName,
+    required this.toUserId,
+    required this.toUserName,
+    required this.amount,
+  });
+}