fix(downloads): carry the active account on the Library poster URL

An <img> can't send the X-Siftlode-Account header, so the poster.jpg request
resolved to the session-default account and _accessible_job 404'd it for any
download owned by a different (per-tab) account — every card then fell back to
the source's remote thumbnail, which for Facebook is an expiring fbcdn URL, so
the Library showed blank thumbnails in a multi-account session. The 0.69.1
local-poster-primary change made this the primary image, exposing it broadly.

Append ?account= to the poster URL via a new api.posterImageUrl(), the same
wallet-gated header-less selection downloadFileUrl / storyboardImageUrl already
use (resolved_user_id reads ?account= for exactly these contexts).

frontend/src/components/DownloadCenter.tsx

@@ -89,7 +89,12 @@ function Thumb({ job }: { job: DownloadJob }) {
   // rule on the source domain can't break it. The source's remote thumbnail_url is only a fallback for
   // the rare poster-less asset (a thumbnail-less source whose ffmpeg frame-cut also failed).
   const remote = job.asset?.thumbnail_url ?? null;
-  const primary = job.poster_url || remote;
+  // `job.poster_url` only SIGNALS a poster exists; the actual <img> src must carry ?account= (see
+  // api.posterImageUrl). A plain <img> can't send the X-Siftlode-Account header, so without it the
+  // poster resolves to the session-default account and 404s for a per-tab account's download — which,
+  // now that the poster is the primary image, reads as a blank Library thumbnail.
+  const poster = job.poster_url ? api.posterImageUrl(job.id) : null;
+  const primary = poster || remote;
   // On load failure (a genuinely missing poster file) fall back to the remote source thumbnail.
   const [src, setSrc] = useState(primary);
   useEffect(() => setSrc(primary), [primary]);