Feature: Preserve spatial context when selecting data points in EmbeddingView

Thank you for sharing and maintaining this excellent project - amazing work!

**Current behavior**
When selecting data points via the Mosaic charts, the EmbeddingView shows only the selected points, and all others disappear. This removes the spatial context, making it difficult to explore the selection within the overall embedding structure.

**Desired behavior**
Show selected points in a highlight color and keep non-selected points visible in grey or muted tones, similar to the Mosaic charts. Optionally, render selected points on top to ensure they remain clearly visible.

**Benefit**
This would make it easier to interpret selections in the spatial context and maintain visual continuity during exploration.

I don’t have a specific implementation approach in mind yet, but happy to clarify the use case further if helpful.

Start by tracing selection handling between the Mosaic charts and EmbeddingView, then inspect how selected and non-selected points are currently rendered. The work is done when selections retain the full embedding, selected points use a highlight treatment, non-selected points remain muted, and selected points stay visibly on top where appropriate.

Written by the indexing model from the issue text.