feat: add unfurl shortcode for rich link preview cards

Adds {% unfurl "URL" %} shortcode that renders any URL as a rich card
with OpenGraph metadata (title, description, image, favicon). Uses
unfurl.js locally — no external API dependency. Results cached for 1
week in .cache/unfurl/. Also fixes Mastodon embed server config
(mstdn.social → indieweb.social).

tailwind.config.js

@@ -7,6 +7,7 @@ export default {
     "./**/*.md",
     "./_includes/**/*.njk",
     "./content/**/*.md",
+    "./lib/**/*.js",
   ],
   darkMode: "class",
   theme: {