seo: prefer per-page metaDescription for meta description tags

~2092 pages set a page-specific metaDescription frontmatter field that
HeadSEO.astro ignored, so almost every page shipped the generic site
tagline as its meta/og/twitter description. Extend the fallback chain to
content.description || content.metaDescription || t('site.description')
for both the description/og:description and twitter:description tags.

This mirrors the description source already used by the JSON-LD in this PR.

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

Author: estradino

src/components/HeadSEO.astro

@@ -52,7 +52,7 @@ const ogLocale = bcpLang.replace(/-/g, '_');
 <meta
 	name="description"
 	property="og:description"
-	content={content.description ? content.description : t('site.description')}
+	content={content.description || content.metaDescription || t('site.description')}
 />
 <meta property="og:site_name" content={t('site.title')} />
 
@@ -62,7 +62,7 @@ const ogLocale = bcpLang.replace(/-/g, '_');
 <meta name="twitter:title" content={content.title ?? t('site.title')} />
 <meta
 	name="twitter:description"
-	content={content.description ? content.description : t('site.description')}
+	content={content.description || content.metaDescription || t('site.description')}
 />
 <meta name="twitter:image" content={canonicalImageSrc} />
 <meta name="twitter:image:alt" content={imageAlt} />