fix(ui): open one session accordion panel at a time

docs/guida-utente.md

@@ -67,7 +67,8 @@ The session sidebar has two accordion sections: **Active sessions** and
 **Archive**, both initially closed. Only their headers appear below the scope tabs.
 Inside each nonempty list, **Select all** selects only that list; its delete action
 also applies only to the selected sessions in that list. The other list's selection
-is preserved. Both sections can be open together; long lists scroll inside
+is preserved. Opening a section closes the other; clicking the open section closes
+it too. Empty lists show only "No sessions yet." Long lists scroll inside
 their own panels. Here active means not archived, not necessarily a running model
 process. Existing groups and session actions remain inside those sections.