Add auxiliaries toggle to line vis toolbar

[axelboc]

Fix #1867

• < NX Line > Allow toggling display of auxiliary signals
• [LineVis] Add prop showAux to control visibility of auxiliaries (defaults to true)

Screencast.from.2025-09-12.10-10-09.webm

For simplicity, I hide the toggle completely when there aren't any auxiliary signals — I don't show it in a disabled state like we do for errors (or rather used to do, see below). The toolbar is a bit packed now and I feel that showing disabled toggles for errors and auxiliaries is no longer worth the space.

• < NX Line > Hide "Errors" toggle completely in toolbar (instead of disabling it) when there aren't any error values
• < Line > Never show "Errors" toggle at all, since only the NX Line visualization can ever have error values

Finally, a small change for convenience, in case consumers just want to plot a line/curve with errors and don't have the need to toggle them off/on:

• [LineVis, DataCurve] If prop showErrors is not specified, default to true

[axelboc]

Superseded by #1911