feat(contracts): drag-to-reorder active contract rows in sidebarcontract-drag-reorder

HTML5 drag/drop on active contract rows. Dragging a row over
another in the same directive folder shows a green top-border drop
indicator; dropping calls reorderDirectiveContract(id, targetPosition)
and refreshes the folder. Shipped/archived rows aren't draggable
(historical, ordering is fixed).

Implementation:
- DocumentRow gains optional draggable + drag event props.
- DirectiveFolder owns the drag/over state and handleReorder
  callback; computes target position from the drop-target row's
  current position.
- Repository's reorder endpoint already exists from the backbone PR
  and handles sibling shift in a transaction.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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