feat(ui): render dependency-chain rail, step badge, and after-chip in TaskRowView

Reuses the existing 24px indent track (now gated on ShowAsChild OR
ShowAsChainMember) instead of a second mechanism, adds a round step-number
badge centered on the rail line, and a dimmed "after X" chip for chain
members whose head isn't in view or that render as a planning child.

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -1045,6 +1045,25 @@
     <Setter Property="Background" Value="{DynamicResource ManualBadgeBrush}"/>
   </Style>
 
+  <!-- ============================================================ -->
+  <!-- CHAIN DEPENDENCY RAIL (step badge on the 24px indent track)  -->
+  <!-- ============================================================ -->
+  <Style Selector="Border.chain-step-badge">
+    <Setter Property="Width"           Value="16"/>
+    <Setter Property="Height"          Value="16"/>
+    <Setter Property="CornerRadius"    Value="{StaticResource PillCornerRadius}"/>
+    <Setter Property="Background"      Value="{StaticResource Surface2Brush}"/>
+    <Setter Property="BorderBrush"     Value="{StaticResource LineBrush}"/>
+    <Setter Property="BorderThickness" Value="1"/>
+  </Style>
+  <Style Selector="Border.chain-step-badge > TextBlock">
+    <Setter Property="FontFamily"          Value="{StaticResource MonoFont}"/>
+    <Setter Property="FontSize"            Value="{StaticResource FontSizeEyebrow}"/>
+    <Setter Property="Foreground"          Value="{StaticResource TextDimBrush}"/>
+    <Setter Property="HorizontalAlignment" Value="Center"/>
+    <Setter Property="VerticalAlignment"   Value="Center"/>
+  </Style>
+
   <!-- ============================================================ -->
   <!-- SHARED MODAL STYLES (promoted from per-modal Window.Styles)  -->
   <!-- ============================================================ -->