refactor(client): centralize error feedback with toast events

- emit API/network failures through a shared toast event bridge

- remove per-page inline error notifications to avoid duplicate error UI

- dedupe repeated toast messages to keep feedback readable

client/src/pages/scenario/ScenariosPage.tsx

@@ -114,7 +114,6 @@ export function ScenariosPage() {
           </Button>
         </div>
       </div>
-      {error && <p className={styles.error}>{error}</p>}
       <Table
         columns={columns}
         data={items}