ui: the sidebar is two tabs, and a toggle disables its pane

Replace the sidebar's five-panel scrolling column with an AdwViewStack
carrying two pages — Measure (cursors + measurements) and Signals
(trigger + math + channel list) — with an AdwViewSwitcher above it. The
trigger and cursors toggles change from "reveal/hide my pane" to
"enable/disable my feature, leaving the pane insensitive", following the
disable-not-hide principle. The keys probe loses a duplicate registration
that was toggling the sidebar back on before the exit probe captured its
state; the exit-probe captures now read adw_overlay_split_view_get_show_
sidebar() instead of gtk_widget_get_visible(), which AdwOverlaySplitView
never changes.

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

src/ui_trigger.cpp

@@ -195,7 +195,9 @@ void on_trigger_toggled(GtkToggleButton* button, gpointer user) {
   if (on && signal >= 0) {
     app->model->set_signal_enabled(static_cast<std::size_t>(signal), true);
   }
-  gtk_revealer_set_reveal_child(GTK_REVEALER(app->trigger_revealer), on);
+  // Disabled, not hidden: a panel you can still see is a panel you can read, and the settings you
+  // are about to switch back on must not vanish while you look for them.
+  gtk_widget_set_sensitive(app->trigger_panel, on);
   // The event-history button is part of trigger mode and the pane it opens is *on demand*; leaving
   // trigger mode takes the button away and collapses the pane, so going back to streaming is not a
   // column of stale events beside the plot. Recording is the other half of that answer, so this is