refactor(ui): clean up task-row drag visuals

Collapse the grabbed row out of layout instead of overlapping neighbors
with scale/opacity/shadow (the ghost window already shows the moving
snapshot). Replace the thin moss drop-hint lines with a dashed
placeholder gap sized like a collapsed row. Suppress the ordinary hover
highlight and transitions on whatever row is currently the drop target
so it doesn't read as "clickable" while dragging.

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -484,12 +484,21 @@
     <Setter Property="BorderBrush"     Value="{StaticResource AccentBrush}" />
     <Setter Property="BorderThickness" Value="1" />
   </Style>
-  <!-- "Grabbed" row: lift + slight scale + lower opacity + shadow while the custom drag runs. -->
+  <!-- "Grabbed" row: collapses out of layout entirely while the custom drag runs — the
+       floating ghost snapshot (TaskDragController) is what the user sees moving. -->
   <Style Selector="Border.task-row.dragging">
-    <Setter Property="Opacity"         Value="0.55" />
-    <Setter Property="RenderTransform" Value="scale(1.03)" />
-    <Setter Property="BoxShadow"       Value="0 10 26 0 #66000000" />
-    <Setter Property="BorderBrush"     Value="{StaticResource AccentBrush}" />
+    <Setter Property="IsVisible"   Value="False" />
+    <Setter Property="Transitions" Value="{x:Null}" />
+  </Style>
+
+  <!-- Row currently hovered while dragging (shows the dashed drop-hint gap): suppress the
+       ordinary hover highlight and transitions so they don't fight the hint. Placed after
+       Border.task-row:pointerover so its extra class also wins on declaration order. -->
+  <Style Selector="Border.task-row.drop-target">
+    <Setter Property="Transitions" Value="{x:Null}" />
+  </Style>
+  <Style Selector="Border.task-row.drop-target:pointerover">
+    <Setter Property="BorderBrush" Value="{StaticResource LineBrush}" />
   </Style>
 
   <!-- Checkbox indicator (the 18px circle that replaces the native CheckBox template) -->