fix: stable GoRouter instance + spinner on dashboard initial load

frontend/lib/main.dart

@@ -16,7 +16,6 @@ import 'screens/ventes_screen.dart';
 import 'screens/achats_screen.dart';
 import 'screens/production_screen.dart';
 import 'screens/stock_screen.dart';
-import 'widgets/app_drawer.dart';
 
 void main() {
   runApp(
@@ -35,15 +34,28 @@ void main() {
   );
 }
 
-class RayhanApp extends StatelessWidget {
+class RayhanApp extends StatefulWidget {
   const RayhanApp({super.key});
 
   @override
-  Widget build(BuildContext context) {
-    final authProvider = context.watch<AuthProvider>();
+  State<RayhanApp> createState() => _RayhanAppState();
+}
 
-    final router = GoRouter(
+class _RayhanAppState extends State<RayhanApp> {
+  late final GoRouter _router;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+  }
+
+  @override
+  void initState() {
+    super.initState();
+    final authProvider = context.read<AuthProvider>();
+    _router = GoRouter(
       initialLocation: '/login',
+      refreshListenable: authProvider,
       redirect: (context, state) {
         final loggedIn = authProvider.isAuthenticated;
         final onLogin = state.matchedLocation == '/login';
@@ -61,7 +73,16 @@ class RayhanApp extends StatelessWidget {
         GoRoute(path: '/stock', builder: (_, __) => const StockScreen()),
       ],
     );
+  }
 
+  @override
+  void dispose() {
+    _router.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
     return MaterialApp.router(
       title: 'Rayhan ERP',
       debugShowCheckedModeBanner: false,
@@ -73,35 +94,7 @@ class RayhanApp extends StatelessWidget {
         useMaterial3: true,
         fontFamily: 'Roboto',
       ),
-      routerConfig: router,
-    );
-  }
-}
-
-class _PlaceholderScreen extends StatelessWidget {
-  final String title;
-  final String route;
-  const _PlaceholderScreen({required this.title, required this.route});
-
-  @override
-  Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(title: Text(title)),
-      drawer: AppDrawer(currentRoute: route),
-      body: Center(
-        child: Column(
-          mainAxisSize: MainAxisSize.min,
-          children: [
-            Icon(Icons.construction_outlined, size: 64, color: Colors.grey[400]),
-            const SizedBox(height: 16),
-            Text('Module $title',
-                style: const TextStyle(fontSize: 18, fontWeight: FontWeight.bold)),
-            const SizedBox(height: 8),
-            Text('En cours de développement',
-                style: TextStyle(color: Colors.grey[600])),
-          ],
-        ),
-      ),
+      routerConfig: _router,
     );
   }
 }

frontend/lib/screens/dashboard_screen.dart

@@ -90,7 +90,7 @@ class _DashboardScreenState extends State<DashboardScreen> {
     }
 
     final kpi = provider.kpi;
-    if (kpi == null) return const SizedBox.shrink();
+    if (kpi == null) return const Center(child: CircularProgressIndicator());
 
     return RefreshIndicator(
       onRefresh: () => context.read<DashboardProvider>().load(),