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/credential/EditCredentialPage.tsx

@@ -84,7 +84,6 @@ export function EditCredentialPage() {
         />
       </div>
 
-      {error && <p className={styles.error}>{error}</p>}
       {loading && <p className={styles.muted}>{t('credentials.loading')}</p>}
 
       {!loading && credential && (