feat(ui): add drag-and-drop reordering for segment rules

[steveninhle]

Summary

• Install @dnd-kit/core and @dnd-kit/sortable for drag-and-drop support
• Wrap the segment rule list in DndContext + SortableContext; each card
  uses useSortable and shows a grip handle (GripVertical) when there are
  multiple rules
• Add DragOverlay so the dragged card follows the cursor with a shadow;
  the original card fades to 30% opacity while dragging
• Keep up/down arrow buttons as an accessible fallback for keyboard users
• Add segmentRulesMove (from useFieldArray) alongside the existing swap,
  and handleMoveSegmentRule in the targeting page to sync both form state
  and featureRef on drag end