feat: decide on drag-to-reorder behavior in stacked mobile layout

[jglopez]

Depends on the stacked row layout issue.

Problem

The table supports drag-to-reorder columns (participants) via @dnd-kit. In the stacked layout, "columns" no longer exist visually, so the drag affordance would either do nothing or behave unexpectedly.

Decision needed for implementer

Pick one approach:

• Disable at breakpoint: hide drag handles below sm. Simplest.
• Reinterpret as slot reorder: allow dragging participant chips in the bottom row to reorder them.
• No change: leave drag logic in place if handles aren't visible in the stacked layout anyway.

Notes

• Drag logic lives in src/components/ItemsTable.tsx via @dnd-kit/sortable.
• useColumnOrder hook manages column order persistence.