Highlight / Scroll mixer options

[eakwarren]

Resolves: #23662

This PR adds two options to the mixer menu: Highlight selection and Scroll to selection.

Kapture.2026-02-23.at.10.24.17.mp4

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[eakwarren]

@avvvvve @jessjwilliamson Please review when you have a moment. Thanks!