Don't show Sample Rate slider with Jack backend

Since in that case the jack server is what dictates the sample rate and format, the slider in Settings is misleading since its value isn't used. Not a fatal issue, but a screaming potential source of user confusion.

Start by locating the Settings sample-rate slider and the code that identifies or configures the Jack backend. Confirm how the selected backend reaches the settings UI. Done means the sample-rate slider is not shown when Jack controls the sample rate and format, while remaining available for other backends.

Written by the indexing model from the issue text.