Playback Controls Now Reflect Active State
Fixed an issue where the inst volume slider appeared on screen during moment-only playback.
During moment-only playback, the inst volume slider remained visible on screen even when no inst track was active. Displaying a control for an inactive audio source introduces ambiguity about the current playback state.
The root cause was that the visibility condition for volume controls was not tied to actual playback state. The fix ensures each control is shown only when its corresponding track is actively playing. When no inst track is present, the inst volume slider is hidden.
Playback controls should accurately reflect what is currently audible. This fix enforces that correspondence.