feat: compelete bal pending screen

lib/ui/bal_page/view_model/bal_view_model.dart

@@ -15,6 +15,22 @@ class BalViewModel extends ChangeNotifier {
   Bal? _bal;
   int? id;
   Bal? get bal => _bal;
+  bool isABalOngoing = false;
+
+  Future<Result<void>> startBal(int id) async {
+    if (isABalOngoing) {
+      return Result.error(Exception("Cannot have multiple BALs ongoing !"));
+    }
+    final result = await _balRepository.startBal(id);
+    switch (result) {
+      case Ok():
+        _bal = result.value;
+        notifyListeners();
+        break;
+      default:
+    }
+    return result;
+  }
 
   Future<Result<void>> editBal(
     int id,
@@ -46,6 +62,7 @@ class BalViewModel extends ChangeNotifier {
   bool isLoaded = false;
 
   Future<Result<void>> _load() async {
+    isABalOngoing = _balRepository.isABalOngoing();
     final result1 = await _loadBal();
     switch (result1) {
       case Ok():

lib/ui/bal_page/widget/pending/bal_pending_screen.dart

@@ -12,6 +12,7 @@ class BalPendingScreen extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final theme = Theme.of(context);
     return Scaffold(
       bottomNavigationBar: AppNavigationBar(startIndex: 0),
       appBar: AppBar(
@@ -31,7 +32,15 @@ class BalPendingScreen extends StatelessWidget {
       ),
       body: Center(
         child: ElevatedButton(
-          onPressed: () {},
+          onPressed: viewModel.isABalOngoing
+              ? () {
+                  ScaffoldMessenger.of(context).showSnackBar(
+                    SnackBar(content: Text("Une BAL est déjà en cours.")),
+                  );
+                }
+              : () {
+                  viewModel.startBal(viewModel.bal!.id);
+                },
           child: Text("Démarrer cette BAL"),
         ),
       ),