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/server/api/articles/[slug].get.ts

@@ -1,6 +1,20 @@
 import type { Article } from '#shared/types/blog'
 
-type RawArticle = Omit<Article, 'html' | 'summary'> & { content: string | null }
+interface AdminUser {
+  firstname: string | null
+  lastname: string | null
+}
+
+type RawArticle = Omit<Article, 'html' | 'summary' | 'author'> & {
+  content: string | null
+  createdBy?: AdminUser | null
+}
+
+/** Joins the admin user's name parts; returns null when neither is set. */
+function byline(user: AdminUser | null | undefined): string | null {
+  const name = [user?.firstname, user?.lastname].filter(Boolean).join(' ').trim()
+  return name || null
+}
 
 export default defineEventHandler(async (event): Promise<Article> => {
   const slug = getRouterParam(event, 'slug')
@@ -19,6 +33,11 @@ export default defineEventHandler(async (event): Promise<Article> => {
     throw createError({ statusCode: 404, statusMessage: 'Article not found' })
   }
 
-  const { content, ...rest } = article
-  return { ...rest, html: renderMarkdown(content), summary: summarise(content) }
+  const { content, createdBy, ...rest } = article
+  return {
+    ...rest,
+    html: renderMarkdown(content),
+    summary: summarise(content),
+    author: byline(createdBy),
+  }
 })