fix: apply configured social colours on hover for follow and share (#861)

The generated per-network hover CSS never matched any networks: it filtered
the social networks map with `where ... "slug" "in"`, which yields nothing for
a Params map, so no per-network rules were emitted and icons kept the default
grey on hover.

Build the rules by ranging the configured network slugs and indexing into the
networks map (as the follow/share partials already do), covering the union of
follow and share networks, and set both `color` and `fill` so the monochrome
SVG icons pick up the configured brand colour on hover.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>