fix(ui): align island header separators via shared header height

src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml

@@ -12,10 +12,12 @@
   <DockPanel>
 
     <!-- ── Header (sticky top): id · title · trash/skull · gear — task detail only ── -->
-    <Border DockPanel.Dock="Top" Classes="island-header"
-            IsVisible="{Binding IsTaskDetailVisible}">
+    <Grid DockPanel.Dock="Top" IsVisible="{Binding IsTaskDetailVisible}">
+      <Grid.RowDefinitions><RowDefinition Height="Auto" SharedSizeGroup="IslandHeader"/></Grid.RowDefinitions>
+      <Border Classes="island-header">
         <detail:TaskHeaderBar/>
       </Border>
+    </Grid>
 
     <!-- ── Body: task details (normal), notes editor (notes mode), or prep log (prep mode) ── -->
     <Grid>

src/ClaudeDo.Ui/Views/Islands/ListsIslandView.axaml

@@ -183,7 +183,9 @@
   <DockPanel LastChildFill="True">
 
     <!-- ── Header ── -->
-    <Border DockPanel.Dock="Top" Classes="island-header">
+    <Grid DockPanel.Dock="Top">
+      <Grid.RowDefinitions><RowDefinition Height="Auto" SharedSizeGroup="IslandHeader"/></Grid.RowDefinitions>
+      <Border Classes="island-header">
         <StackPanel Spacing="4">
           <TextBlock Classes="heading" Text="{loc:Tr lists.heading}"/>
   
@@ -204,6 +206,7 @@
           </Border>
         </StackPanel>
       </Border>
+    </Grid>
 
     <!-- ── No-repo banner — visible until any User list has a linked working dir; no
          dismiss, no persisted state, comes back if the last repo link is removed. ── -->

src/ClaudeDo.Ui/Views/Islands/TasksIslandView.axaml

@@ -10,7 +10,9 @@
   <DockPanel LastChildFill="True">
 
     <!-- Header -->
-    <Border DockPanel.Dock="Top" Classes="island-header">
+    <Grid DockPanel.Dock="Top">
+      <Grid.RowDefinitions><RowDefinition Height="Auto" SharedSizeGroup="IslandHeader"/></Grid.RowDefinitions>
+      <Border Classes="island-header">
         <Grid ColumnDefinitions="*,Auto">
           <StackPanel Grid.Column="0" Spacing="4">
             <TextBlock Classes="eyebrow" Text="{Binding HeaderEyebrow}"/>
@@ -68,6 +70,7 @@
           </StackPanel>
         </Grid>
       </Border>
+    </Grid>
 
     <!-- Add-task row -->
     <Border DockPanel.Dock="Top" Classes="add-task" Margin="16,14,16,8"

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -142,7 +142,8 @@
     </Border>
 
     <!-- Three islands (user-resizable) -->
-    <Grid Grid.Row="1" Margin="7">
+    <!-- Shared-size scope: the island headers share one height so their separators line up -->
+    <Grid Grid.Row="1" Margin="7" Grid.IsSharedSizeScope="True">
       <Grid.ColumnDefinitions>
         <ColumnDefinition Width="260" MinWidth="200"/>
         <ColumnDefinition Width="Auto"/>