Take a thumbnail out of the video when the listed one is gone

A favorite outlives its thumbnail. Sources sign their CDN paths and the
signature expires, so a video kept months ago has a `thumb` that 404s and
a card that is a grey box with a title under it. Asking the source page
for a fresh picture covers the case where the page still exists; nothing
covered the case where it doesn't.

The video does, because playback re-resolves from the page URL every time.
So once every route to the listed picture is exhausted, a frame a quarter
of the way in becomes the card's picture -- past the black lead-in, and a
fair guess at what the video looks like. It goes through our own proxy, so
the canvas can be read back, is scaled to card width and JPEG'd to a
handful of KB, and is kept: later mounts and later sessions paint it
without a request, instead of asking again for a picture that is known to
be gone.

Only for favorites, once per item, one at a time, and never for a live
stream or a manifest. Two things worth knowing, both found with the
browser rather than reasoned about: a browser fires `seeked` for its own
initial seek to zero, so taking the frame on the first one captures the
opening of every video; and the same item is on screen in more than one
place, so the captures are deduped by a promise rather than the first
asker getting the picture and the rest being told there isn't one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MPZFnNdHbPGDTqQUNiE4ZN

frontend/js/favorites.js

@@ -494,7 +494,7 @@ App.favorites = App.favorites || {};
             `;
             const thumb = card.querySelector('img');
             if (App.videos && typeof App.videos.attachThumbnail === 'function') {
-                App.videos.attachThumbnail(thumb, item.thumb, App.videos.sourcePageUrl(item));
+                App.videos.attachThumbnail(thumb, item.thumb, App.videos.sourcePageUrl(item), item);
             }
             card.onclick = () => {
                 if (card.classList.contains('is-loading')) return;