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/ScenarioDetailPage.tsx

@@ -11,7 +11,6 @@ import {
   Card,
   DescriptionList,
   Input,
-  Notification,
   Select,
   Table,
   Timestamp,
@@ -306,14 +305,6 @@ export function ScenarioDetailPage() {
         )}
       </div>
 
-      {error && (
-        <Notification
-          variant="error"
-          title={error.startsWith('404') ? t('errors.not_found') : undefined}
-        >
-          {error.startsWith('404') ? t('errors.not_found_scenario', { id }) : error}
-        </Notification>
-      )}
       {loading && <p className={styles.muted}>{t('scenarios.loading')}</p>}
 
       {scenario && (