Add a "Site Frontpage" linkit matcher

Problem/Motivation

Editors can't easily insert a link to a site's frontpage in CKEditor/link fields — there's no linkit suggestion for it.

Proposed resolution

• New site_frontpage linkit matcher: searches sites by label, inserts a [site_id] token.
• SitesUrlLinkitFilter resolves [site_id] / [site_id]path tokens to the site's absolute frontpage URL on render. The existing entity-link path is unchanged; token resolution runs as a separate narrow pass over non-entity links only (lower-risk than rewriting process() to scan every link).