Decouple volume slider and volume slider scroll sensitivities

[Sv443]

Currently the scroll sensitivity is rounded to whatever nearest value fits the regular "click" sensitivity.
A fix would be to always set the step to 1 and round it to the respective setting manually.
This way the volume steps could also be changed without having to reload the page.

I still need to investigate the viability however, since there could be outside factors like the volume being controlled by external apps via the Media Session API or PiP windows.