Support for highlighting path between selected nodes

Hi team,

I found myself recently needing to investigate the phylogenetic distance between 2 specific ASVs and thought it might be really useful if Empress could visually highlight the full path between 2 (or more?) user-selected nodes. Apologies if this functionality already exists, couldn't find a way to get what I wanted.

The issue names no files or tests. Start by reproducing selection of two ASVs in Empress and reviewing the existing tree-node selection behavior; confirm how a path should be highlighted and whether selecting more than two nodes is in scope. Done means the agreed selection flow visibly highlights the requested path.

Written by the indexing model from the issue text.