Show the author and publication time on articles

Strapi already records which admin user created an entry, so the byline needs
no extra field to fill in: the Article content type sets populateCreatorFields
and the API layer exposes only that user's firstname and lastname. The name
also feeds the BlogPosting structured data.

Also fixes the gap below the article header, which until now came from the
cover image's margin and therefore vanished on articles without a cover,
leaving the body text against the byline.

frontend/shared/types/blog.ts

@@ -26,6 +26,8 @@ export interface Article extends ArticleSummary {
   html: string
   /** Plain-text opening of the body, used as the meta description. */
   summary: string
+  /** Byline: the full name of the admin user who wrote the article. */
+  author: string | null
 }
 
 export interface Paginated<T> {