Scroll mixer left and right when moving through selected channels

Nobody has claimed this yet.

### Enhancement Summary

When the mixer has many channels, they can be gone through using the left/right arrow keys and moved using alt-left/alt-right.
But switching right through the channels and going out of the view of the mixer does not scroll the mixer to the right to show the selected channel as we switch, and similarly with the left.
Similarly, using alt-left/right does not scroll the mixer to show tracks out of view.

### Implementation Details / Mockup

Current behaviour:

https://github.com/user-attachments/assets/3ab15bd2-d33e-4f9d-9e40-a64b4bd52b79

(Initially selecting rightward with arrow keys, and then switching mixer channels with alt-right... sorry about the stuttering video.)
Notice how the mixer will not scroll to show the current selected channel even as we move it.

**NOTE: This should only happen when using these actions, the selected channel should not be forced to stay in view constantly**

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

The issue names no files or tests; start by locating the mixer’s channel-selection handlers for left/right and alt-left/alt-right actions, then reproduce the behavior with more channels than fit in view. Done means those actions scroll the mixer to reveal the selected or moved channel in either direction, without forcing the selected channel to remain visible during other interactions.

Written by the indexing model from the issue text.