Homepage: show 5 featured plates, brand CTA button

Mark five posts (Gothic, Cyberpunk, Dark Fantasy, Urban, Sci-Fi) as
featured: true. Homepage grid now filters by that flag instead of
showing first 8. CTA button changed from plain black to brand russet.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

content/posts/gothic-cathedral-red-gown/index.md

@@ -6,6 +6,7 @@ slug: "gothic-cathedral-red-gown"
 issues:
   - "01"
 image: "gothic-cathedral-red-gown.png"
+featured: true
 weight: 33
 categories:
   - "Gothic"