Keep mention hover icons from reflowing following text
The previous version animated the icon pseudo-elements' width 0 -> 0.85em on
hover, which pushed any text after the mention sideways (only unnoticed because
the test mention ended its paragraph). Reserve the icon width + margin permanently
(invisible at rest via opacity 0) and animate only opacity + transform on hover —
neither affects layout, so following text stays put. Same technique the author
handle link uses. Color stays var(--sun)/currentColor, i.e. the per-publication
theme accent. Verified in-browser: trailing text holds its x-position between rest
and hover while both icons fade in.