Brain GUI modernization (Phase 6)

[payamsash]

Reference issue (if any)

Phase 5 of #14042 and following #14149

What does this implement/fix?

• the peak vertex now changes per overlay, and the activity trace is rescaled respectively
• the peak sphere now is more glossy

Additional information

• Claude helped by reviewing the changes and the tests to ensure that I didn't miss anything.
• the trace vertical time line is not draggable anymore ... however this issue is not related to this PR, might be coming from Brain GUI modernization (Phase 5) #14149 ... will look at it in the next PRs

[larsoner]

Just pushed a couple of small fixes, marking for merge-when-green, thanks in advance @payamsash !