Implement Optimistic UI Updates

const addTodo = useMutation({
     mutationFn: (text) => fetch('/api/todos', { method: 'POST', body: JSON.stringify({ text }) }),
     onMutate: async (newTodo) => {
       await queryClient.cancelQueries({ queryKey: ['todos'] });
       const previous = queryClient.getQueryData(['todos']);
       queryClient.setQueryData(['todos'], (old) => [...old, newTodo]);
       return { previous };
     },
     onError: (err, newTodo, context) => {
       queryClient.setQueryData(['todos'], context.previous);
     },
   });