Footer storage/backend display and a staff role that actually grants staff

Footer:
- Left is now 'Backend Storage:' with a capacity bar (blue, peach at 80%,
  red at 95% per DESIGN.md) and a used/total/free tooltip; the watched
  folder path is no longer printed. /api/status/ returns a compact storage
  summary instead of the path (the full storage page still shows paths to
  authenticated users).
- Centre shows the backend API origin (empty = same origin). Staff get a
  link to /setup to point the browser elsewhere; everyone else sees it as
  plain text. The Account 'Backend connection' card is gone — this is
  installation plumbing, not a per-user setting.
- Design spec updated to match.

Staff role:
- The custom role did nothing on several endpoints that only accepted
  Django's is_staff/is_superuser. One canonical check now exists:
  User.is_app_staff (superuser, Django staff, or the staff role), used by
  the stats/users APIs, item object permissions, can_delete, upload/
  similarity/download/match querysets, and the management commands
  (which also pick staff-role accounts for e621 sync/match and file
  ownership).

Verified with a role-only staff account (is_staff/is_superuser false):
stats/users 200, all 32 downloads + 2 scans visible, others' items
editable; the same account as role=user gets 403 for all of those.

frontend/src/features/account/AccountPage.tsx

@@ -14,7 +14,6 @@ import { useE621 } from "@/store/e621";
 import { toast } from "@/store/toasts";
 
 import { AvatarCard } from "./AvatarCard";
-import { BackendCard } from "./BackendCard";
 import { PreferencesCard } from "./PreferencesCard";
 
 const BASE_URL_OPTIONS = [
@@ -216,7 +215,6 @@ export default function AccountPage() {
         <>
           <AvatarCard />
           <PreferencesCard />
-          <BackendCard />
         </>
       ) : null}
       {loading && !credentials ? (