fix(ST-511): read create-state errors in Story dialog fieldError

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

components/backlog/story-dialog.tsx

@@ -98,7 +98,8 @@ export function StoryDialog({ state, onClose, isDemo = false }: StoryDialogProps
   )
 
   const fieldError = (field: string) => {
-    const err = updateResult?.error
+    const result = isEdit ? updateResult : createResult
+    const err = result?.error
     if (!err || typeof err === 'string') return undefined
     return (err as Record<string, string[]>)[field]?.[0]
   }