update tagline to canonical line from about.md

Generic 'Trending Topics' replaced with the explicit positioning
line from about.md:

  "Where the internet's MVPs go viral"

Flows through to <meta description>, og:description, twitter:description
and the visible header tagline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/site.js

@@ -1,6 +1,6 @@
 export const site = {
   name: 'ViralMVP',
-  description: 'Trending Topics',
+  description: "Where the internet's MVPs go viral",
   url: 'https://viralmvp.com',
   postsPerPage: 10,
 };