/**
 * Two-way state sync across browser tabs.
 * - Without callback: returns [value, setter] (like useState but synced).
 * - With callback: calls your handler when any tab updates this key.
 *
 * While the socket is null the setter updates local state only; it broadcasts
 * once the socket is ready.
 *
 * @example
 * const [cart, setCart] = useSocketSync<Cart>('cart', { items: [] });
 */
export declare function useSocketSync<T>(key: string, initialValue: T, callback?: (value: T) => void): [T, (value: T) => void];