feat(files): the Downloads page goes — the tray, a Storage tab and a Downloads settings tab take its place

- The Downloads module, its rail icon and badge are gone. A bookmark, the remembered page and an old
  history entry that name it land on Files (urlState.resolvePage); `g d` goes to Files too.
- Users page: a Storage tab — the disk in numbers, every user's use against their own limit with
  their file count, every user's downloads; aligned tables, one menu per row (edit / reset quota,
  open source, copy, the owner's quota). The admin storage rows carry quota + file count, the admin
  job list its owner.
- Settings: a Downloads tab — the download formats in one list (duplicate a built-in, edit or delete
  your own) and the site sign-ins, moved from Account.
- "View" on an added download, a clip or a converting upload opens the tray on that row (unfolded,
  back if closed; a finished one opens as a file; under a player it opens without taking focus).
- The download dialog opens on the format used last.
- DataTable: `pager={false}` for a short list, `cardRow` to give a lone "…" its own card line.
- Welcome and README speak of Files; a new Files screenshot.

frontend/src/components/LibraryPlayer.tsx

@@ -36,7 +36,7 @@ export default function LibraryPlayer({
   const qc = useQueryClient();
   const fileUrl = useMemo(() => api.fileUrl(job.id), [job.id]);
   // Video vs audio-only comes from the stored media_kind (the single source every sibling reads —
-  // WatchPage/downloadRow/DownloadCenter) — NOT a re-derived container list, which would misclassify
+  // WatchPage/downloadRow/the tray) — NOT a re-derived container list, which would misclassify
   // an uploaded audio file whose container is outside a hardcoded set (uploads accept any container).
   // Only fall back to the old dimension+container heuristic for a legacy asset with a null media_kind.
   const asset = job.asset;