feat: the session file remembers which sidebar tab was showing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui_layout.cpp

@@ -58,6 +58,7 @@ LayoutState collect_layout(App& app) {
   st.math = app.math;
   st.measures = app.measures;
   st.measure_stats = app.measure_stats_on;
+  st.side_tab = app.side_tab;
   // The connection is the session's too: the file's name carries only host and port.
   st.multicast = app.source_multicast;
   st.interface_addr = app.source_interface;
@@ -327,6 +328,7 @@ void apply_layout(App& app, const LayoutState& st) {
   app.measures = st.measures;
   app.measure_stats_on = st.measure_stats;
   app.updating_panels = true;
+  select_side_tab(app, st.side_tab.c_str());
   if (app.measures_stats_check != nullptr) {
     gtk_check_button_set_active(GTK_CHECK_BUTTON(app.measures_stats_check),
                                 app.measure_stats_on);