Function useShowItem

Master-detail dispatch helper. Selects the item in the attention context, then — based on the current layout mode — shows its detail surface:

• 'mobile': expand the complementary sidebar on the given companion segment.
• deck modes ('solo'/'multi'): open the item as a sibling plank beside the master (pivotId = contextId), when a path is provided.
• otherwise: swap the current plank's companion to the given segment.