enhancement: Config validation allows MaxOutputBytes=0 and MaxDownloadBytes=0

## Summary

Config validation checks `< 0` for `MaxOutputBytes` and `MaxDownloadBytes`, which allows `0`. A value of `0` effectively disables all output and downloads — almost certainly a misconfiguration.

## Location

`config/config.go:186-193`

## Suggested Fix

Either change the check to `<= 0` or add a warning when zero values are configured.