The husband died before the marriage was consummated: must the wife still observe the ʿiddah?

Yes. The ʿiddah of death of 4 months 10 days is obligatory even if the marriage was not consummated. This differs from divorce: a wife divorced before consummation has no ʿiddah. The verse on the ʿiddah of death applies generally to all wives whose husbands die, and its purpose is not only to ensure the womb is empty but also to honour the marriage bond. This wife also inherits from her husband and is entitled to the full mahr. She must observe mourning (ihdad) throughout the ʿiddah. Once the ʿiddah ends, she is free to remarry. See ʿiddah of death.