Fix palette drag-and-drop not triggering AddModuleModal

DevicePalette's useDraggable was missing dragType: 'palette' in its data
object. RackPlanner's handleDragStart and handleDragEnd both guard on
dragType === 'palette' — without it the drag overlay never showed and the
drop onto a slot was silently ignored.

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

client/src/components/rack/DevicePalette.tsx

@@ -16,7 +16,7 @@ const ALL_TYPES: ModuleType[] = [
 function PaletteItem({ type }: { type: ModuleType }) {
   const { attributes, listeners, setNodeRef, isDragging } = useDraggable({
     id: `palette-${type}`,
-    data: { type },
+    data: { dragType: 'palette', type },
   });
 
   const colors = MODULE_TYPE_COLORS[type];