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/locales/en.json

@@ -40,7 +40,8 @@
     "images": "Images",
     "documents": "Documents",
     "fonts": "Fonts",
-    "ebooks": "Ebooks"
+    "ebooks": "Ebooks",
+    "archives": "Archives"
   },
   "footer": {
     "rights": "Ombrora Convert — Online file conversion"