ui(properties): sub-headings sit flush left, level with the category header

Their rank is carried by size and colour, not by an indent — so Video, Sound byte and Ticker all
line up on the left edge exactly like each other, and only the controls under them tab in.

Themes/Controls.xaml

@@ -159,11 +159,13 @@
         <Setter Property="Margin" Value="0,0,0,6"/>
     </Style>
 
+    <!-- Sub-headings sit FLUSH LEFT, level with the category header; it is their smaller size and
+         dimmer colour that make them subordinate, not an indent. Only the controls indent. -->
     <Style x:Key="PropertySubHeader" TargetType="TextBlock">
         <Setter Property="Foreground" Value="{StaticResource Brush.OnSurfaceVariant}"/>
         <Setter Property="FontSize" Value="10"/>
         <Setter Property="FontWeight" Value="SemiBold"/>
-        <Setter Property="Margin" Value="10,6,0,3"/>
+        <Setter Property="Margin" Value="0,6,0,3"/>
     </Style>
 
     <!-- A field label that sits in a row with its control (DockPanel.Dock="Left", fixed Width).