Function
metadataBase is what turns a relative og:url or og:image into the
absolute URL crawlers require. Without it Next emits the relative path, the
page renders fine, and every share preview silently loses its image.
title.default covers routes that set no title of their own; a route that
does set one gets %s · English Course from the template.
Spec: site-metadata § "resolves one absolute site URL", lesson-view-polish § "The Lesson Page sets a per-page
Per-locale defaults every route inherits: the origin its relative URLs are absolutized against, and the title template that appends the brand.