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.

backend/apps/follows/sync.py

@@ -9,6 +9,7 @@ user following it.
 import logging
 
 from django.contrib.auth import get_user_model
+from django.db.models import Q
 from django.utils import timezone
 
 from apps.library import e621
@@ -32,7 +33,9 @@ def preferred_fetch_user(username=None):
     if username:
         return User.objects.filter(username=username).first()
     return (
-        User.objects.filter(is_staff=True)
+        User.objects.filter(
+            Q(is_superuser=True) | Q(is_staff=True) | Q(role=User.ROLE_STAFF)
+        )
         .exclude(e621_username="")
         .exclude(e621_api_key="")
         .order_by("id")