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/modules.ts

@@ -3,7 +3,6 @@ import {
   BarChart3,
   Bell,
   Clapperboard,
-  Download,
   Home,
   ListVideo,
   ScrollText,
@@ -95,13 +94,6 @@ const MODULES: readonly ModuleDef[] = [
     railKey: "nav.rail.files",
     icon: FolderOpen,
   },
-  {
-    id: "downloads",
-    labelKey: "downloads.navLabel",
-    railKey: "nav.rail.downloads",
-    icon: Download,
-    hasBadge: true,
-  },
   {
     id: "stats",
     labelKey: "header.account.stats",