At what distance does Proxima Centauri become visible to the naked eye?
"Back of an envelope" calculation:

Proxima Centauri has an apparent magnitude of about $11$. The faintest objects visible to the unaided eye have magnitudes about $6.5$. So we need to decrease Proxima Centauri's magnitude by about $5$ in round numbers, which corresponds to an increase in brightness of about $100$ times. To achieve this we would have to be about $10$ times closer to Proxima Centuari than we are now, which gives a distance of about $0.4$ ly.

At a distance of $0.75$ ly the apparent magnitude of Proxima Centauri would be about $7.5$, so I think Celestia is being a little optimistic.