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/tableColumns.ts

@@ -37,6 +37,9 @@ export interface Column<T> {
   cardPrimary?: boolean;
   hideInCard?: boolean;
   cardLabel?: boolean;
+  // …and `cardRow` gives the value a line of its own: a row's lone "…" flowed in after the facts,
+  // at the end of one card's line and on the next line of another — never in the same place twice.
+  cardRow?: boolean;
   // A column a table WITHOUT the card fallback (the playlist's drag-reorderable hybrid) leaves out
   // while the TABLE is narrower than 42rem: its fixed widths alone overran the box and left the title
   // no width. The table's own width, not the viewport's (an `@container` round it): beside the