fix: harden offline fallback and track visitable pages

src/i18n/messages/en/common.json

@@ -144,10 +144,11 @@
       "initializing": "Initializing...",
       "totalStorage": "Total storage",
       "imagesQuota": "{used}% of images quota used",
+      "visitablePages": "{count} pages available offline",
       "static": "Static resources",
       "staticDesc": "Next.js scripts, styles and assets",
       "pages": "Visited pages",
-      "pagesDesc": "Home, libraries, series and details",
+      "pagesDesc": "Pages and navigation data available offline",
       "api": "API data",
       "apiDesc": "Series, books and library metadata",
       "images": "Images",