Support custom screen snapshot save dir

## Description

Currently, the app saves screen snapshots as a downloaded file and does not provide an option for the user to specify a custom save directory. This feature request is to add the ability for the user to specify a custom directory for saving screen snapshots.

## Solution

Support a new option in config to specify a custom directory for saving snapshots.

No files, tests, or entry points are named. Start by locating the configuration handling and the code that saves downloaded screen snapshots; done means a configured custom directory is accepted and snapshots are saved there instead of the default location.

Written by the indexing model from the issue text.