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/AdminUsers.tsx

@@ -52,8 +52,10 @@ import { MoreButton } from "./ui/MoreButton";
 import type { ActionMenuItem } from "./ui/ActionMenu";
 import { Input } from "./ui/Input";
 import { SURFACE } from "./ui/surfaces";
+import { StorageTab } from "./downloadAdmin";
 
-// Admin-only user & access management: roles and access requests (the Invite whitelist). Each
+// Admin-only user & access management: roles, access requests (the Invite whitelist), and the
+// storage every user's downloads take (with their quotas). Each
 // section is its own tab (persisted across reloads); the Access tab carries a badge with the count
 // of pending requests so it's visible without switching to it.
 // (The tab key + a "focus the Access tab" helper live in lib/adminUsersTab so callers can
@@ -81,6 +83,8 @@ export default function AdminUsers() {
       label: t("users.tabs.access"),
       badge: me.pending_invites + me.pending_activations,
     },
+    // What every user's downloads take on the disk, against their own limit (the quota editor).
+    { id: "storage", label: t("users.tabs.storage") },
   ];
   const active = tabs.some((x) => x.id === tab) ? tab : "roles";
   return (
@@ -99,6 +103,7 @@ export default function AdminUsers() {
             <PendingAccess />
           </>
         )}
+        {active === "storage" && <StorageTab />}
       </div>
     </>
   );