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/CreateScenarioPage.tsx

@@ -48,8 +48,6 @@ export function CreateScenarioPage() {
         />
       </div>
 
-      {error && <p className={styles.error}>{error}</p>}
-
       <form onSubmit={handleSubmit} noValidate>
         <Card className={styles.formCard}>
           <div className={styles.formFields}>