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/ViewModels/Islands/TaskRowViewModel.cs

@@ -45,6 +45,10 @@ public sealed partial class TaskRowViewModel : ViewModelBase
     // Set by the custom drag while this row is being dragged — drives the "grabbed" row style.
     [ObservableProperty] private bool _isDragging;
 
+    // True while a drag is hovering this row (i.e. it would show a drop-hint gap). Used to
+    // suppress the ordinary hover highlight/transitions so they don't fight the hint.
+    public bool IsDropTarget => DropHintAbove || DropHintBelow;
+
     public bool CanRefine => Status == TaskStatus.Idle && PlanningPhase == PlanningPhase.None
                              && !IsRefining && !IsManual;
 
@@ -289,6 +293,8 @@ public sealed partial class TaskRowViewModel : ViewModelBase
     partial void OnDiffAdditionsChanged(int value) { OnPropertyChanged(nameof(HasDiff)); OnPropertyChanged(nameof(DiffAdditionsText)); }
     partial void OnDiffDeletionsChanged(int value) { OnPropertyChanged(nameof(HasDiff)); OnPropertyChanged(nameof(DiffDeletionsText)); }
     partial void OnRoadblockCountChanged(int value) { OnPropertyChanged(nameof(HasRoadblock)); OnPropertyChanged(nameof(RoadblockTooltip)); }
+    partial void OnDropHintAboveChanged(bool value) => OnPropertyChanged(nameof(IsDropTarget));
+    partial void OnDropHintBelowChanged(bool value) => OnPropertyChanged(nameof(IsDropTarget));
 
     public void RefreshLocalized()
     {