feat: implement shared LoadingModal component and integrate into dashboard route loading states

src/app/(dashboard)/stock-negatif/loading.tsx

@@ -0,0 +1,5 @@
+import LoadingModal from "@/components/shared/loading-modal";
+
+export default function StockNegatifLoading() {
+    return <LoadingModal message="Chargement des données" subMessage="Récupération des stocks négatifs..." />;
+}