Do not render in AddDetail when visibility is disabled

[AdamMagued]

Summary

When a progress bar is configured with OptionSetVisibility(false), calling AddDetail would inadvertently render the progress bar to the output writer if state.details was nil. In addition, ProgressBar.render() lacked an explicit check for config.invisible.

Root Cause

In progressbar.go, AddDetail attempts to render the progress bar prior to attaching the initial detail row if p.state.details == nil. However, this call to p.render() was executed unconditionally without checking p.config.invisible. Furthermore, p.render() itself did not guard against p.config.invisible like other methods (Add64, RenderBlank, Describe, renderDetails).

Changes

1. Guard p.render() call in AddDetail with if !p.config.invisible.
2. Guard render() entry point with if p.config.invisible { return nil }.
3. Add unit tests in progressbar_test.go covering AddDetail both when visible and when configured with OptionSetVisibility(false) to ensure no bytes are written to the writer.

Fixes #236

[AdamMagued]

Closing in favor of earlier community PR #237 to avoid duplicate review effort.