feat: the session file remembers which sidebar tab was showing

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

src/layout_store.hpp

@@ -66,6 +66,10 @@ struct LayoutState {
   /// stream and mean nothing restored against another one.
   bool measure_stats = false;
 
+  /// Which sidebar tab was showing: "measure" or "signals". The one thing the panel persists --
+  /// no revealer states remain, and the 320 px width is the `.ui`'s.
+  std::string side_tab = "signals";
+
   /// The measurements pane, by signal *name* and with the type and options a row was built with.
   std::vector<Measure> measures;