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/components/count/expenses_tab.dart

@@ -0,0 +1,142 @@
+import 'package:flutter/material.dart';
+import 'package:intl/intl.dart';
+import '../../data/models/expense.dart';
+import '../../data/models/group.dart';
+import 'expense_detail_dialog.dart';
+
+class ExpensesTab extends StatelessWidget {
+  final List<Expense> expenses;
+  final Group group;
+
+  const ExpensesTab({
+    super.key,
+    required this.expenses,
+    required this.group,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    if (expenses.isEmpty) {
+      return const Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Icon(Icons.receipt_long, size: 80, color: Colors.grey),
+            SizedBox(height: 16),
+            Text(
+              'Aucune dépense',
+              style: TextStyle(fontSize: 20, fontWeight: FontWeight.bold),
+            ),
+            SizedBox(height: 8),
+            Text(
+              'Ajoutez votre première dépense',
+              style: TextStyle(fontSize: 14, color: Colors.grey),
+            ),
+          ],
+        ),
+      );
+    }
+
+    return ListView.builder(
+      padding: const EdgeInsets.all(16),
+      itemCount: expenses.length,
+      itemBuilder: (context, index) {
+        final expense = expenses[index];
+        return _buildExpenseCard(context, expense);
+      },
+    );
+  }
+
+  Widget _buildExpenseCard(BuildContext context, Expense expense) {
+    final isDark = Theme.of(context).brightness == Brightness.dark;
+    final dateFormat = DateFormat('dd/MM/yyyy');
+
+    return Card(
+      margin: const EdgeInsets.only(bottom: 12),
+      child: InkWell(
+        onTap: () => _showExpenseDetail(context, expense),
+        borderRadius: BorderRadius.circular(12),
+        child: Padding(
+          padding: const EdgeInsets.all(16),
+          child: Row(
+            children: [
+              Container(
+                width: 48,
+                height: 48,
+                decoration: BoxDecoration(
+                  color: isDark ? Colors.blue[900] : Colors.blue[100],
+                  borderRadius: BorderRadius.circular(12),
+                ),
+                child: Icon(
+                  expense.category.icon,
+                  color: Colors.blue,
+                ),
+              ),
+              const SizedBox(width: 16),
+              Expanded(
+                child: Column(
+                  crossAxisAlignment: CrossAxisAlignment.start,
+                  children: [
+                    Text(
+                      expense.description,
+                      style: const TextStyle(
+                        fontSize: 16,
+                        fontWeight: FontWeight.bold,
+                      ),
+                    ),
+                    const SizedBox(height: 4),
+                    Text(
+                      'Payé par ${expense.paidByName}',
+                      style: TextStyle(
+                        fontSize: 14,
+                        color: isDark ? Colors.grey[400] : Colors.grey[600],
+                      ),
+                    ),
+                    Text(
+                      dateFormat.format(expense.date),
+                      style: TextStyle(
+                        fontSize: 12,
+                        color: isDark ? Colors.grey[500] : Colors.grey[500],
+                      ),
+                    ),
+                  ],
+                ),
+              ),
+              Column(
+                crossAxisAlignment: CrossAxisAlignment.end,
+                children: [
+                  Text(
+                    '${expense.amount.toStringAsFixed(2)} ${expense.currency.symbol}',
+                    style: const TextStyle(
+                      fontSize: 16,
+                      fontWeight: FontWeight.bold,
+                      color: Colors.green,
+                    ),
+                  ),
+                  if (expense.currency != ExpenseCurrency.eur)
+                    Text(
+                      '${expense.amountInEur.toStringAsFixed(2)} €',
+                      style: TextStyle(
+                        fontSize: 12,
+                        color: isDark ? Colors.grey[400] : Colors.grey[600],
+                      ),
+                    ),
+                ],
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+
+  void _showExpenseDetail(BuildContext context, Expense expense) {
+    showDialog(
+      context: context,
+      builder: (context) => ExpenseDetailDialog(
+        expense: expense,
+        group: group,
+      ),
+    );
+  }
+}