Sync EmbeddingView selection with global selectedIndices and show trails for all selected points

[Weiyu-Kong]

Current Behavior

• On the EmbeddingView canvas, I can multi-select points by holding Ctrl/Cmd and left-clicking multiple points (area 1 in the screenshot).
• However, the Selected panel does not display any of these selected points (area 2).
• When I check the showTrail option (area 3), only the point currently hovered or clicked shows its motion trail (area 4), instead of all selected points.

Root Cause

The selected points on the EmbeddingView canvas (available via the onChange callback) are not synchronized with our global selectedIndices state.

Tasks

1. When multiple points are selected on the canvas, keep selectedIndices in sync with the canvas selection (update selectedIndices accordingly).
2. When showTrail is checked, render motion trails for all selected points, not just the currently hovered/clicked one.
3. (Advanced) When a point is deselected from the Selected panel (e.g., by clicking the “x” icon), also deselect that point on the canvas. In general, ensure the EmbeddingView selection and the global selectedIndices stay fully consistent in both directions.

Related Code and Doc

• https://apple.github.io/embedding-atlas/embedding-view.html
• web/src/component/chart.tsx
• web/src/state/state.unified.ts
• web/src/component/function-panel.tsx