Function toggle

frontend/src/utils/sets.ts:34–42

* Return a new set with `item` toggled — removed if present, added if not.

(set: Set<T>, item: T)

   * Return a new set with `item` toggled — removed if present, added if not.
   */
  toggle<T>(set: Set<T>, item: T): Set<T> {
    const result = new Set(set);
    if (result.has(item)) {
      result.delete(item);
    } else {
      result.add(item);
    }
    return result;
  },
};