Is it legislated for a woman observing the 'iddah (waiting period) after her husband's death to fast the entire 'iddah period out of obedience to Allah Almighty, and what is the ruling on dedicating the reward of this fasting to her deceased husband?

It is permissible for a woman in her 'iddah (waiting period) to fast the entire 'iddah voluntarily for the sake of Allah, except on the days of Eid, the days of Tashreeq, and the days of menstruation. She should fast Ramadan with the intention of fulfilling the obligatory fast. It is also sunnah for her to make abundant supplications for her deceased husband, and it is permissible for her to give charity on his behalf. However, voluntary fasting on his behalf is a matter of differing opinions.