ui: the Dock toggle carries the ruler icon, the dock being the readouts

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

src/ui/header-bar.ui

@@ -46,8 +46,8 @@
     </child>
     <child type="end">
       <object class="GtkToggleButton" id="sidebar_toggle">
-        <property name="icon-name">sidebar-show-symbolic</property>
-        <property name="tooltip-text" translatable="yes">show the dock</property>
+        <property name="icon-name">udpscope-measure-symbolic</property>
+        <property name="tooltip-text" translatable="yes">show the readouts dock (cursors, measurements, events)</property>
         <property name="active">1</property>
       </object>
     </child>