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/main-window.ui

@@ -3,7 +3,7 @@
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
 <!--
   The main window's skeleton: the window, the split view that puts the sidebar beside the plot, the
-  events pane's divider, the plot column with the status line under it, and the sidebar's scroller.
+  events pane's divider, the plot column with the status line under it, and the sidebar's tab stack.
 
   The sidebar is an `AdwOverlaySplitView`, which is what a utility sidebar is in GNOME (as against
   `AdwNavigationSplitView`, which is for navigation lists). Three things follow from that, and they are
@@ -20,7 +20,7 @@
   What is *not* here is as deliberate as what is. Everything this file nests is fixed — the shape of
   the window does not depend on the stream, the trigger or the layout — while the subtrees that do are
   built by code and put in place: the events pane, into `events_divider`'s start side; the two plot
-  areas, into the split inside `plot_box`; the sidebar's panels, appended to `side_column`; and the
+  areas, into the split inside `plot_box`; the sidebar's panels, appended to `measure_page` and `signals_page`; and the
   status popover, attached to `status_info`.
 
   The header bar is not here either: it is the window's own titlebar, built by `build_toolbar()`
@@ -48,18 +48,63 @@
         <property name="min-sidebar-width">320</property>
         <property name="max-sidebar-width">320</property>
         <property name="sidebar">
-          <!-- The column scrolls as a whole, so a long signal list or a revealed trigger panel can
-               never push either panel out of the window. -->
-          <object class="GtkScrolledWindow" id="side_scroll">
-            <property name="hscrollbar-policy">never</property>
-            <property name="vscrollbar-policy">automatic</property>
-            <property name="min-content-width">300</property>
-            <property name="child">
-              <object class="GtkBox" id="side_column">
-                <property name="orientation">vertical</property>
-                <property name="width-request">300</property>
+          <!-- A switcher above a stack, and a scroller *per page*: the two pages are different
+               heights, so one shared scroller around the stack would leave a tab switch scrolled
+               to somewhere the new page never asked for. The switcher is the sibling above, not a
+               child of either scroller: the one control that chooses what you are looking at must
+               not be the first thing to scroll away. -->
+          <object class="GtkBox" id="side_column">
+            <property name="orientation">vertical</property>
+            <property name="width-request">300</property>
+            <child>
+              <object class="AdwViewSwitcher" id="side_switcher">
+                <property name="stack">side_stack</property>
+                <property name="policy">wide</property>
+                <property name="margin-top">4</property>
+                <property name="margin-bottom">4</property>
+                <property name="margin-start">6</property>
+                <property name="margin-end">6</property>
               </object>
-            </property>
+            </child>
+            <child>
+              <object class="AdwViewStack" id="side_stack">
+                <property name="vexpand">1</property>
+                <child>
+                  <object class="AdwViewStackPage">
+                    <property name="name">measure</property>
+                    <property name="title" translatable="yes">Measure</property>
+                    <property name="child">
+                      <object class="GtkScrolledWindow">
+                        <property name="hscrollbar-policy">never</property>
+                        <property name="vscrollbar-policy">automatic</property>
+                        <property name="child">
+                          <object class="GtkBox" id="measure_page">
+                            <property name="orientation">vertical</property>
+                          </object>
+                        </property>
+                      </object>
+                    </property>
+                  </object>
+                </child>
+                <child>
+                  <object class="AdwViewStackPage">
+                    <property name="name">signals</property>
+                    <property name="title" translatable="yes">Signals</property>
+                    <property name="child">
+                      <object class="GtkScrolledWindow">
+                        <property name="hscrollbar-policy">never</property>
+                        <property name="vscrollbar-policy">automatic</property>
+                        <property name="child">
+                          <object class="GtkBox" id="signals_page">
+                            <property name="orientation">vertical</property>
+                          </object>
+                        </property>
+                      </object>
+                    </property>
+                  </object>
+                </child>
+              </object>
+            </child>
           </object>
         </property>
         <property name="content">