do you remember the a thousand years war audio drama when Alleria has a vision of the tiranny of the light where people is put in stasis for ever in some sort of crystals?? and remember the crystal tecnology of the naaru in their ships??, i think is safe to assume those crystals are permanently charged by the light and work as an infinite source of energy, thats why the people trapped in the crystals never die, because the light is keeping them alive.
I think the body of the lightforged is in fact an artifitial body made of millions of tiny versions of these crystals and thats why a lightforged can live for ever.
But what does this has to do with Calia?? well, because i think the body of a lightforged can be shaped in any way possible, you just need to arrange the crystals in a different order, so i think the naaru made her look like an undead as a dirty political move to trick the undead into joining the light.