Enable changing exo models directory via macOS app

[AlexCheema]

Currently we have an environment variable to control the model directory (see https://github.com/exo-explore/exo/blob/main/src/exo/shared/constants.py). However, there's no way to set this via the macOS app.

It would be great if this was possible to set at runtime instead of requiring an app restart see: https://github.com/exo-explore/exo/issues/1008