Save result screenshots on disk instead of encoding it in the JSON

https://github.com/gbdev/GBEmulatorShootout/blob/main/main.py#L221

Storing screenshots as files keeps the JSON lightweight and directly usable as a data source for any frontend (React, Vue, etc.). Image paths are trivially renderable/serve-able

Start at main.py#L221 and trace how result screenshots are currently placed in the JSON. Check how result data is produced and consumed, then change the output so screenshots are stored as files and the JSON contains usable image paths. Done means the JSON is lighter and the saved paths can be rendered or served by a frontend.

Written by the indexing model from the issue text.