fix: add class to unfurl fallback link to prevent YouTube double-embed

When unfurl cache is failed/missing for a YouTube URL, renderFallbackLink
returned a classless <a> which matched the youtube-link-to-embed transform
regex (the class exclusion from 819aab1 didn't protect it). This created
one embed from the custom transform and a second from eleventy-plugin-youtube-embed.

Adding class="unfurl-fallback" triggers the (?![^>]*\bclass=) exclusion,
preventing both embeds on like/repost pages with uncached YouTube URLs.

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

lib/unfurl-shortcode.js

@@ -76,7 +76,7 @@ export function escapeHtml(str) {
 
 export function renderFallbackLink(url) {
   const domain = escapeHtml(extractDomain(url));
-  return `<a href="${escapeHtml(url)}" rel="noopener" target="_blank">${domain}</a>`;
+  return `<a href="${escapeHtml(url)}" rel="noopener" target="_blank" class="unfurl-fallback">${domain}</a>`;
 }
 
 export function renderCard(url, metadata) {