fix: initialize fr_FR/fr_TN locale data before runApp (DateFormat crash)

frontend/lib/main.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import 'package:go_router/go_router.dart';
+import 'package:intl/date_symbol_data_local.dart';
 
 import 'providers/auth_provider.dart';
 import 'providers/dashboard_provider.dart';
@@ -17,7 +18,10 @@ import 'screens/achats_screen.dart';
 import 'screens/production_screen.dart';
 import 'screens/stock_screen.dart';
 
-void main() {
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await initializeDateFormatting('fr_FR');
+  await initializeDateFormatting('fr_TN');
   runApp(
     MultiProvider(
       providers: [
@@ -42,22 +46,18 @@ class RayhanApp extends StatefulWidget {
 }
 
 class _RayhanAppState extends State<RayhanApp> {
-  late final GoRouter _router;
+  GoRouter? _router;
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-  }
-
-  @override
-  void initState() {
-    super.initState();
-    final authProvider = context.read<AuthProvider>();
+    if (_router == null) {
+      final auth = Provider.of<AuthProvider>(context, listen: false);
       _router = GoRouter(
         initialLocation: '/login',
-      refreshListenable: authProvider,
-      redirect: (context, state) {
-        final loggedIn = authProvider.isAuthenticated;
+        refreshListenable: auth,
+        redirect: (_, state) {
+          final loggedIn = auth.isAuthenticated;
           final onLogin = state.matchedLocation == '/login';
           if (!loggedIn && !onLogin) return '/login';
           if (loggedIn && onLogin) return '/dashboard';
@@ -74,15 +74,17 @@ class _RayhanAppState extends State<RayhanApp> {
         ],
       );
     }
+  }
 
   @override
   void dispose() {
-    _router.dispose();
+    _router?.dispose();
     super.dispose();
   }
 
   @override
   Widget build(BuildContext context) {
+    if (_router == null) return const SizedBox.shrink();
     return MaterialApp.router(
       title: 'Rayhan ERP',
       debugShowCheckedModeBanner: false,
@@ -94,7 +96,7 @@ class _RayhanAppState extends State<RayhanApp> {
         useMaterial3: true,
         fontFamily: 'Roboto',
       ),
-      routerConfig: _router,
+      routerConfig: _router!,
     );
   }
 }