feat(frontend): add archive format family, compression control, and fixed double-extension parsing

Also adds a --color-family-archives CSS variable (light + dark) that
the plan didn't call out explicitly but is needed for the new family
tile/icon to render with a color instead of falling back silently.

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

frontend/src/pages/HomePage.jsx

@@ -9,6 +9,7 @@ import { ReassuranceStrip } from '../components/ReassuranceStrip.jsx';
 import { FormatsGrid } from '../components/FormatsGrid.jsx';
 import { FormatMarquee } from '../components/FormatMarquee.jsx';
 import { SeoHead } from '../components/SeoHead.jsx';
+import { extensionOf } from '../utils/archiveExtensions.js';
 import '../styles/home.css';
 import '../styles/sections.css';
 
@@ -19,14 +20,15 @@ const DEFAULT_QUALITY = {
   avif: 50,
   tiff: 80,
   png: 6,
+  zip: 6,
+  'tar.gz': 6,
+  'tar.bz2': 9,
+  '7z': 5,
+  'tar.7z': 5,
 };
 
 const DEFAULT_ICON_SIZE = 256;
 
-function extensionOf(fileName) {
-  return fileName.split('.').pop().toLowerCase();
-}
-
 function defaultQualityFor(targetFormat) {
   return DEFAULT_QUALITY[targetFormat] ?? null;
 }