Add drag-to-reorder to the packing and to-do lists, mirroring the budget
panel's native HTML5 drag pattern. A drag within a filtered/grouped view is
mapped back onto the global order so untouched items keep their place, and the
order persists optimistically via the existing reorder endpoints.
Packing items can now be marked private (#858): a private item is visible only
to its owner. createItem/bulkImport stamp the owner, listItems filters by the
viewer, and the WebSocket broadcasts are scoped to the owner so a private item
never reaches another member's screen — including the public/private toggle
transitions. Owners get a lock toggle and a private indicator on their items.
Maurice
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