Is it permissible for the treasurer of the mosque to buy food and drinks for the worshippers from the charity funds designated for the mosque, or should these funds be allocated only to pay for rent and water?

The intent of the donors must be considered. If they intended a specific purpose, such as paying rent, the money must be spent on that. If their intent was merely an act of worship (qurbah), then there is no objection to using it to feed those who perform night prayers (mutahajjidin) what sustains them in their worship, provided there is no extravagance.