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/lib/format.ts

@@ -45,12 +45,6 @@ export function formatBytes(bytes: number | null | undefined, lang: string): str
   return `${num} ${units[i]}`;
 }
 
-/** Download speed from bytes/sec, e.g. "2.4 MB/s". */
-export function formatSpeed(bps: number | null | undefined, lang: string): string {
-  if (!bps) return "";
-  return `${formatBytes(bps, lang)}/s`;
-}
-
 /** Relative-time tiers as [seconds-per-unit, Intl unit]. The unit is what the divisor produces,
  * and Intl.RelativeTimeFormat turns the pair into the localized words — so a divisor can no
  * longer be paired with the wrong label, which is exactly what shifted every tier by one from