[Feature] Add all new features to selectedMultiVar

[davehorsfall]

Is your feature request related to a problem? Please describe.
When viewing a Plotly-based multi-variable plot (e.g., matrix, dot, violin, heatmap) and adding new features, all features are added to selectedMultiVar, which works as expected. However, if features were previously added from the Scatterplot, those same features may not always be pre-selected when switching to a multi-variable plot.

This behavior appears inconsistent. Sometimes the previously selected features are included, and other times they aren’t. Based on testing, this inconsistency seems related to timing: if the features are selected quickly (before the debounce completes), they might not be added properly?

Describe the solution you'd like
When adding new features to the list in a multi-variable plot, all selected features should be pre-selected by default. This provides a more predictable and user-friendly experience when switching between plot types.

That said, I’m open to discussion - if automatic pre-selection isn’t the desired UX, the key point is that the behavior should at least be consistent and reproducible, regardless of how quickly users select variables from the search modal.