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/moduleRegistry.tsx

@@ -28,7 +28,6 @@ const PlexSidebar = lazyWithPreload(() => import("./PlexSidebar"));
 const NotificationsPanel = lazyWithPreload(() => import("./NotificationsPanel"));
 const Files = lazyWithPreload(() => import("./Files"));
 const FilesRail = lazyWithPreload(() => import("./FilesRail"));
-const DownloadCenter = lazyWithPreload(() => import("./DownloadCenter"));
 const Stats = lazyWithPreload(() => import("./Stats"));
 const Scheduler = lazyWithPreload(() => import("./Scheduler"));
 const ConfigPanel = lazyWithPreload(() => import("./ConfigPanel"));
@@ -57,7 +56,6 @@ export const PAGE_CONTENT: Record<Page, PageDef> = {
   plex: { Component: PlexBrowse, maxWidth: "100rem" },
   notifications: { Component: NotificationsPanel, maxWidth: "48rem" },
   files: { Component: Files, maxWidth: "80rem" },
-  downloads: { Component: DownloadCenter, maxWidth: "56rem" },
   stats: { Component: Stats },
   scheduler: { Component: Scheduler },
   config: { Component: ConfigPanel },