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/Welcome.tsx

@@ -7,7 +7,7 @@ import {
   Clapperboard,
   Clock,
   Code2,
-  Download,
+  FolderOpen,
   Expand,
   Filter,
   Inbox,
@@ -182,7 +182,7 @@ const GALLERY: { key: string; src: string }[] = [
   { key: "playlists", src: "/welcome/playlists.webp" },
   { key: "search", src: "/welcome/search.webp" },
   { key: "player", src: "/welcome/player.webp" },
-  { key: "downloads", src: "/welcome/downloads.webp" },
+  { key: "files", src: "/welcome/files.webp" },
   { key: "plex", src: "/welcome/plex.webp" },
 ];
 
@@ -192,7 +192,7 @@ const FEATURES: { key: string; icon: LucideIcon }[] = [
   { key: "search", icon: Search },
   { key: "playlists", icon: ListVideo },
   { key: "player", icon: PlayCircle },
-  { key: "downloads", icon: Download },
+  { key: "files", icon: FolderOpen },
   { key: "plex", icon: Clapperboard },
   { key: "private", icon: Lock },
 ];