fix: only prompt for credentials encryption when creds will be saved

[NWuensche]

The encryption prompt for user_credentials.json was shown unconditionally
in save_config, even when the user selected 'Save user configuration'.
That option only writes user_configuration.json, so the credentials file
was never created despite the misleading prompt. Gate the encryption
prompt behind 'user_creds' and 'all' save options.