Add column reordering to work item boards

What does this MR do and why?

Add column reordering to work item boards

Let users reorder board columns on the work items planning view, either by dragging a column by its header or via a 'Move left'/'Move right' header actions menu. The order persists per view in the existing displaySettings JSON as a bounded array of group ids, sibling to visibleGroups/collapsedGroups.

The order reconciles on read (unknown/new groups fall to the end in default order, stale ids are ignored), so the stored value never needs rewriting when statuses change. Reorders reflect optimistically and persist through the same path as the other display settings: directly for all-items, and via the saved-view draft + 'Save changes' flow.

Behind the planning_view_boards feature flag.

Changelog: added

References

Related to #607836 (closed) #603866 (closed)

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How to set up and validate locally

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