feat: show actual (absolute) playback rate in the popup

[WofWca]

For example draw it in a corner of the chart.

With the option added in ce1976dd1313332ba519e362d5414e2b9a1a94f5 it can be useful for users to see the actual behavior, maybe the actual speed is different from the expected one.

Or maybe the extension should check it by itself and suggest enabling this option.