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.

tests/ClaudeDo.Worker.Tests/UiVm/TaskRowViewModelTests.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using ClaudeDo.Data.Models;
 using ClaudeDo.Ui.ViewModels.Islands;
 using Xunit;
@@ -20,4 +21,25 @@ public class TaskRowViewModelTests
         vm.Status = s;
         Assert.Equal(expected, vm.StatusChipClass);
     }
+
+    [Fact]
+    public void IsDropTarget_Follows_Either_DropHint_And_Raises_PropertyChanged()
+    {
+        var vm = new TaskRowViewModel { Id = "t" };
+        var raised = new List<string?>();
+        vm.PropertyChanged += (_, e) => raised.Add(e.PropertyName);
+
+        Assert.False(vm.IsDropTarget);
+
+        vm.DropHintAbove = true;
+        Assert.True(vm.IsDropTarget);
+        Assert.Contains(nameof(TaskRowViewModel.IsDropTarget), raised);
+
+        vm.DropHintAbove = false;
+        vm.DropHintBelow = true;
+        Assert.True(vm.IsDropTarget);
+
+        vm.DropHintBelow = false;
+        Assert.False(vm.IsDropTarget);
+    }
 }