fix(desktop): scroll to bottom when switching to active session

Closes #683 — switching to a session whose agent is still
streaming left the viewport pinned near the user's last message
instead of jumping to the latest draft. Mirror the existing
refreshEmptyState(sess); scrollBottom(true) pattern from
sibling render paths so setActiveSession lands at the bottom.

frontends/desktop/static/app.js

@@ -2711,6 +2711,7 @@ function setActiveSession(id) {
   r.draftEl = null;
   resetTypewriterState(r);
   renderAllMessages(sess);
+  scrollBottom(true);
   setBusy(sess, rt(sess).busy);
   renderSessionList();
   refreshPlanBar(null);