Reveal Bluesky + external-link icons on article mentions on hover
Match the profile-page author handle link more fully: on hover/focus the mention
brightens to --sun and a Bluesky logo (left) and external-arrow (right) slide in.
They're drawn as currentColor mask pseudo-elements (::before/::after) because the
mention arrives as sanitised HTML and can't carry nested <svg> children; the icons
inherit the link colour and collapse to zero width at rest so the mention reads as
plain inline text. The sanitiser already gives the anchor target="_blank" + safe rel,
so mentions open in a new tab. Verified in the browser (hover reveals both icons in
--sun).