The list-item-plus-label slide-in needs a visible anchor element so
the button takes up consistent width and the absolutely-positioned
label can fly in to the left of it. Email uses the '+' SVG as that
anchor; here we use an empty 13x13 spacer span instead — same
footprint, no glyph. Result: empty button at rest (still visible per
the chats-manage-btn fade rules), 'manage' slides in from the left
on direct hover.
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