feat: the session file remembers which sidebar tab was showing

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

src/layout_store.cpp

@@ -496,6 +496,8 @@ bool load_layout(const std::string& path, LayoutState& out) {
         }
       } else if (key == "measure_stats") {
         st.measure_stats = value == "true";
+      } else if (key == "side_tab") {
+        st.side_tab = strip_quote(value);
       } else if (key == "multicast") {
         st.multicast = strip_quote(value);
       } else if (key == "interface") {
@@ -645,6 +647,7 @@ bool save_layout(const std::string& path, const LayoutState& state) {
     o << "]\n";
   }
   o << "measure_stats = " << (state.measure_stats ? "true" : "false") << "\n";
+  o << "side_tab = \"" << state.side_tab << "\"\n";
   if (!state.comment.empty()) o << "comment = " << toml_escape(state.comment) << "\n";
   if (state.buffer_mb > 0.0) o << "buffer_mb = " << state.buffer_mb << "\n";
   o << "relative_time = " << (state.relative_time ? "true" : "false") << "\n";