feat(downloads): show container/format chip in library rows (E7 S1)

The completed download rows already surfaced size + duration; add the concrete
container (MP4/M4A/WebM…) as a compact chip so the library reads as a media list
with a visible format — the audio-vs-video distinction the running-only phase pill
can't show at rest. Pure display of the already-serialized asset.container; locked
by a small DOM test (present when ready, absent before the worker fills it).

frontend/src/components/downloadRow.tsx

@@ -215,6 +215,11 @@ export function DownloadRow({
         <ChannelLine job={job} />
         <div className="flex items-center gap-2 mt-1 text-xs">
           <StatusPill job={job} />
+          {job.asset?.container ? (
+            // Concrete container (mp4 / m4a / webm …) — reads as the file format, and its audio-vs-video
+            // extension is the video/audio distinction the running-only phase pill can't show at rest.
+            <span className="text-muted uppercase">· {job.asset.container}</span>
+          ) : null}
           {job.asset?.size_bytes ? (
             <span className="text-muted">· {formatBytes(job.asset.size_bytes)}</span>
           ) : null}