feat(frontend): improve application feedback messages

eme-frontend/lib/screens/restitution_select_screen.dart

@@ -7,6 +7,7 @@ import "../services/emprunt_service.dart";
 import "../services/materiel_service.dart";
 import "../widgets/ensup_top_bar.dart";
 import "../widgets/brand_corners.dart";
+import "../widgets/app_message.dart";
 import "checklist_retour_screen.dart";
 
 /// Sélection de l'emprunt en cours à restituer (RG15 : ses propres emprunts).
@@ -50,9 +51,7 @@ class _RestitutionSelectScreenState extends State<RestitutionSelectScreen> {
       if (!mounted) {
         return;
       }
-      ScaffoldMessenger.of(
-        context,
-      ).showSnackBar(SnackBar(content: Text(error.toString())));
+      AppMessage.erreur(context, error);
     } finally {
       if (mounted) {
         setState(() => _chargementId = null);