Show icon in the tree when a session has an active script running

When running the application from a session via a custom run script (e.g. `npm run`) it would be nice to reflect that somehow in the session tree.

Start by locating how sessions are represented and rendered in the session tree, then trace where custom run scripts such as `npm run` are started. Define the active-script state and display an icon for that session while it runs; done means the icon appears during execution and disappears when the script ends.

索引モデルが issue の本文から書いたものです。