Fix module drag-and-drop: custom collision detection to hit slots over racks

Root cause: SortableContext registers each rack column as a droppable.
Each column is ~1800px tall (42U x 44px). The default closestCenter
algorithm compared center-to-center distances, so the rack column's
center consistently beat the 44px RackSlot's center — meaning over.data
resolved to { dragType: 'rack' } and handleDragEnd's check for
dropType === 'slot' never matched. Drops silently did nothing.

Fix: replace closestCenter with a two-phase collision detection:
  1. pointerWithin — returns droppables whose bounding rect contains
     the actual pointer position. Slots are exactly hit-tested.
  2. closestCenter fallback — used when the pointer is not within any
     registered droppable (e.g. dragging a rack header between columns
     for sortable reorder where the pointer may be in the gap).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/components/rack/RackPlanner.tsx

@@ -3,10 +3,13 @@ import {
   DndContext,
   DragOverlay,
   PointerSensor,
+  pointerWithin,
+  closestCenter,
   useSensor,
   useSensors,
   type DragStartEvent,
   type DragEndEvent,
+  type CollisionDetection,
 } from '@dnd-kit/core';
 import { SortableContext, horizontalListSortingStrategy, arrayMove } from '@dnd-kit/sortable';
 import { toast } from 'sonner';
@@ -50,6 +53,23 @@ function ModuleDragOverlay({ label }: { label: string }) {
   );
 }
 
+/**
+ * Collision detection strategy:
+ * - pointerWithin first: finds droppables whose rect contains the pointer.
+ *   This correctly hits the 44px-tall RackSlots since the pointer is inside them.
+ * - closestCenter fallback: used when the pointer is not within any droppable,
+ *   which is when dragging a rack header between columns (sortable reorder).
+ *
+ * Without this, the default closestCenter alone would favour the large rack-column
+ * sortable elements (~1800px tall) over the tiny slot droppables, so over.data
+ * would be { dragType: 'rack' } instead of { dropType: 'slot' }.
+ */
+const slotFirstCollision: CollisionDetection = (args) => {
+  const within = pointerWithin(args);
+  if (within.length > 0) return within;
+  return closestCenter(args);
+};
+
 export function RackPlanner() {
   const { racks, loading, fetchRacks, moveModule, updateRack } = useRackStore();
   const canvasRef = useRef<HTMLDivElement>(null);
@@ -143,7 +163,7 @@ export function RackPlanner() {
   const rackIds = racks.map((r) => r.id);
 
   return (
-    <DndContext sensors={sensors} onDragStart={handleDragStart} onDragEnd={handleDragEnd}>
+    <DndContext sensors={sensors} collisionDetection={slotFirstCollision} onDragStart={handleDragStart} onDragEnd={handleDragEnd}>
       <div className="flex flex-col h-screen bg-[#0f1117]">
         <RackToolbar rackCanvasRef={canvasRef} />