I was looking for that too. I'm sure I've read that single cell animals were sensitive to light (and/or heat). I guess it's a speculation though, because we'd have no physical evidence.
We know that modern flagellates can steer to or away from light. When they started doing that is, as you say, pretty difficult to establish since they haven't left archaeological evidence. Unlike shellfish.
As another poster has said, most unicellular eukaryotes are either attracted or repelled by light and this is very simple to observe.
For some of them the mechanism of photoreception is partially understood, because they use photosensitive molecules related to those used in the eyes of animals.
Moreover, there are many bacteria that can sense light, so they are also attracted or repelled by light. Again, many of them use rhodopsins for sensing light, the same as our eyes.
Some light-sensitive molecules, like rhodopsins, are known to have existed in living beings for at least a few billion years, so the ability to sense light is that old.
Where the animals have innovated is in developing optical systems that can capture 2-dimensional images, instead of just sensing whether light is present or absent, like the other living beings can do.
