feat(frontend): dashboard KPI complet avec navigation drawer

- DashboardProvider + DashboardService (GET /api/dashboard)
- Modèle DashboardKpi (Ventes, Achats, Production, Stock)
- DashboardScreen : grilles KpiCard, alertes stock, pull-to-refresh
- KpiCard widget réutilisable (icône, couleur, valeur, sous-titre)
- AppDrawer : navigation complète avec surbrillance route active
- Placeholders pour modules Article/Ventes/Achats/Production/Stock
- Rapport PFE : section 5.5 Dashboard (architecture, KPIs, UI)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/providers/dashboard_provider.dart

@@ -0,0 +1,27 @@
+import 'package:flutter/material.dart';
+import '../models/dashboard_kpi.dart';
+import '../services/dashboard_service.dart';
+
+class DashboardProvider extends ChangeNotifier {
+  DashboardKpi? _kpi;
+  bool _isLoading = false;
+  String? _error;
+
+  DashboardKpi? get kpi => _kpi;
+  bool get isLoading => _isLoading;
+  String? get error => _error;
+
+  Future<void> load() async {
+    _isLoading = true;
+    _error = null;
+    notifyListeners();
+    try {
+      _kpi = await DashboardService.fetchKpis();
+    } catch (_) {
+      _error = 'Impossible de charger les données. Vérifiez la connexion.';
+    } finally {
+      _isLoading = false;
+      notifyListeners();
+    }
+  }
+}