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/repositories/count_repository.dart

@@ -0,0 +1,165 @@
+import 'package:cloud_firestore/cloud_firestore.dart';
+import 'package:firebase_storage/firebase_storage.dart';
+import 'dart:io';
+import '../data/models/expense.dart';
+
+class CountRepository {
+  final FirebaseFirestore _firestore;
+  final FirebaseStorage _storage;
+
+  CountRepository({
+    FirebaseFirestore? firestore,
+    FirebaseStorage? storage,
+  })  : _firestore = firestore ?? FirebaseFirestore.instance,
+        _storage = storage ?? FirebaseStorage.instance;
+
+  // Créer une dépense
+  Future<String> createExpense(Expense expense) async {
+    final docRef = await _firestore
+        .collection('groups')
+        .doc(expense.groupId)
+        .collection('expenses')
+        .add(expense.toMap());
+    
+    return docRef.id;
+  }
+
+  // Mettre à jour une dépense
+  Future<void> updateExpense(String groupId, Expense expense) async {
+    await _firestore
+        .collection('groups')
+        .doc(groupId)
+        .collection('expenses')
+        .doc(expense.id)
+        .update(expense.toMap());
+  }
+
+  // Supprimer une dépense
+  Future<void> deleteExpense(String groupId, String expenseId) async {
+    final expense = await _firestore
+        .collection('groups')
+        .doc(groupId)
+        .collection('expenses')
+        .doc(expenseId)
+        .get();
+
+    final data = expense.data();
+    if (data != null && data['receiptUrl'] != null) {
+      await deleteReceipt(data['receiptUrl']);
+    }
+
+    await _firestore
+        .collection('groups')
+        .doc(groupId)
+        .collection('expenses')
+        .doc(expenseId)
+        .delete();
+  }
+
+  // Archiver une dépense
+  Future<void> archiveExpense(String groupId, String expenseId) async {
+    await _firestore
+        .collection('groups')
+        .doc(groupId)
+        .collection('expenses')
+        .doc(expenseId)
+        .update({'isArchived': true});
+  }
+
+  // Marquer une split comme payée
+  Future<void> markSplitAsPaid({
+    required String groupId,
+    required String expenseId,
+    required String userId,
+  }) async {
+    final doc = await _firestore
+        .collection('groups')
+        .doc(groupId)
+        .collection('expenses')
+        .doc(expenseId)
+        .get();
+
+    final expense = Expense.fromFirestore(doc);
+    final updatedSplits = expense.splits.map((split) {
+      if (split.userId == userId) {
+        return ExpenseSplit(
+          userId: split.userId,
+          userName: split.userName,
+          amount: split.amount,
+          isPaid: true,
+        );
+      }
+      return split;
+    }).toList();
+
+    await _firestore
+        .collection('groups')
+        .doc(groupId)
+        .collection('expenses')
+        .doc(expenseId)
+        .update({
+      'splits': updatedSplits.map((s) => s.toMap()).toList(),
+    });
+  }
+
+  // Stream des dépenses d'un groupe
+  Stream<List<Expense>> getExpensesStream(String groupId, {bool includeArchived = false}) {
+    Query query = _firestore
+        .collection('groups')
+        .doc(groupId)
+        .collection('expenses')
+        .orderBy('date', descending: true);
+
+    if (!includeArchived) {
+      query = query.where('isArchived', isEqualTo: false);
+    }
+
+    return query.snapshots().map((snapshot) {
+      return snapshot.docs.map((doc) => Expense.fromFirestore(doc)).toList();
+    });
+  }
+
+  // Uploader un reçu
+  Future<String> uploadReceipt(String groupId, String expenseId, File imageFile) async {
+    final fileName = 'receipts/$groupId/$expenseId/${DateTime.now().millisecondsSinceEpoch}.jpg';
+    final ref = _storage.ref().child(fileName);
+    
+    final uploadTask = await ref.putFile(imageFile);
+    return await uploadTask.ref.getDownloadURL();
+  }
+
+  // Supprimer un reçu
+  Future<void> deleteReceipt(String receiptUrl) async {
+    try {
+      final ref = _storage.refFromURL(receiptUrl);
+      await ref.delete();
+    } catch (e) {
+      // Le fichier n'existe peut-être plus
+      print('Erreur lors de la suppression du reçu: $e');
+    }
+  }
+
+  // Obtenir les taux de change (API externe ou valeurs fixes)
+  Future<Map<ExpenseCurrency, double>> getExchangeRates() async {
+    // TODO: Intégrer une API de taux de change réels
+    // Pour l'instant, valeurs approximatives
+    return {
+      ExpenseCurrency.eur: 1.0,
+      ExpenseCurrency.usd: 0.92,
+      ExpenseCurrency.gbp: 1.17,
+      ExpenseCurrency.jpy: 0.0062,
+      ExpenseCurrency.chf: 1.05,
+      ExpenseCurrency.cad: 0.68,
+      ExpenseCurrency.aud: 0.61,
+    };
+  }
+
+  // Convertir un montant en EUR
+  Future<double> convertToEur(double amount, ExpenseCurrency currency) async {
+    if (currency == ExpenseCurrency.eur) return amount;
+    
+    final rates = await getExchangeRates();
+    final rate = rates[currency] ?? 1.0;
+    return amount * rate;
+  }
+}