feat(frontend): add archive pair to the homepage format marquee

Every other family (images, documents, fonts, ebooks) had a showcase
pair in the marquee; archives was missing one after the archive
conversion feature landed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/FormatMarquee.jsx

@@ -6,6 +6,7 @@ const PAIRS = [
   ['docx', 'pdf'],
   ['ttf', 'woff'],
   ['epub', 'mobi'],
+  ['rar', 'zip'],
 ];
 
 function Pill({ from, to }) {