Advanced AI Prompt for Next.js 15 Optimistic UI with Server Actions

Act as an Elite UX Frontend Engineer. Architect a zero-latency Optimistic UI pattern in Next.js 15 leveraging the `useOptimistic` hook paired tightly with a complex [MUTATION_ACTION] Server Action. For the deeply nested [UI_STATE_OBJECT] (e.g., a multi-item shopping cart), write the precise React logic to instantly mutate the client DOM the millisecond the user clicks, completely masking the network latency. Provide the rigorous `try/catch` implementation required inside the form action to forcefully execute the [ERROR_ROLLBACK] logic, instantaneously reverting the UI state and displaying a toast notification if the Server Action encounters a 500 database error.