- When dragging a selected card while multiple cards are selected, the
browser ghost image now shows a 'N cards' badge instead of a single card.
- All selected cards in the original column are dimmed (opacity 0.45 +
grayscale) during the drag so the user sees the whole set is in-flight.
- Uses React state for the dragged task id; event delegation on the board
columns container to avoid deep prop threading.
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