Collect discovered video metadata and recommend videos from listening history

frontend/sw.js

@@ -60,6 +60,7 @@ const SHELL = [
   '/refresh.css',
   '/theme-glass.css',
   '/theme-classic.css',
+  '/recommendations.css',
   '/flag-ui.js',
   '/export.js',
   '/async-guard.js',
@@ -203,6 +204,12 @@ self.addEventListener('fetch', (e) => {
   // The admin page is online-only and must never be served from a cache.
   if (url.pathname === '/admin' || url.pathname.startsWith('/admin.')) return;
 
+  // Catalog artwork uses the same offline image cache as YouTube artwork.
+  if (url.origin === self.location.origin && /^\/api\/catalog\/[\w-]{11}\/thumbnail$/.test(url.pathname)) {
+    e.respondWith(thumbnail(request));
+    return;
+  }
+
   // API calls and binary downloads → network only, no caching
   if (url.pathname.startsWith('/api/')) {
     e.respondWith(fetch(request));