Support switching Codex proxy / provider

Hi, thanks for building CC Mate.

I’d like to request support for switching Codex proxy / provider in the UI.

For users who use different Codex endpoints for different regions, teams, or accounts, switching currently requires manual config changes. It would be great if CC Mate could support:
- saving multiple Codex proxy/provider configurations
- switching between them easily
- optionally testing connection before switching

This would make multi-environment usage much more convenient.

Thanks!