feat(frontend): dedupe /public/settings via shared usePublicSettings hook (#325)

Pre-dedup: 7 calls to /api/public/settings on a single /admin/login page
load — 4 from raw-fetch consumers + 3 from React Query consumers using
inconsistent queryKeys. Captured live in Chrome DevTools.

Post-dedup: 1 call. Verified by tests/e2e/public-settings-dedup.spec.ts.

Adds:
- frontend/src/hooks/usePublicSettings.ts — single React Query hook,
  60s staleTime, queryKey ['public-settings']. Vitest with mocked api
  proves multi-mount dedup.
- Extended PublicSettings interface with seo_meta_* fields used by
  RobotsMetaTags and branding_logo_* fields used by GalleryView/AdminHeader.

Migrates 19 call sites across 4 risk-ordered rounds:
- Round A (high fan-in): GlobalThemeProvider, MaintenanceContext (with
  refetchInterval to preserve maintenance polling), MaintenanceWrapper
  (drops the now-redundant per-route ping; axios interceptor already
  handles 503), AdminHeader.
- Round B (gallery/login): GalleryView, GalleryPage, ClientAccessPage,
  AdminLoginPage, MaintenanceMode.
- Round C (decorative): RobotsMetaTags, DynamicFavicon, CMSContentBlock,
  ReCaptcha, useWatermarkSettings (rips out raw fetch + local state),
  LegalPage.
- Round D (queryKey alignment): useLocalizedDate, UserPhotoUpload,
  CreateEventPage. EventDetailsPage Round D ships in the follow-up
  commit that adds presigned-download UI on the same page.

App.tsx (AnalyticsBootstrap) and EventDetailsPage are deferred to
later commits — both files mix #325 changes with backend feature work.

frontend/src/components/admin/AdminHeader.tsx

@@ -9,11 +9,12 @@ import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { useAdminAuth } from '../../contexts';
 import { useAdminDarkMode } from '../../contexts/AdminDarkModeContext';
 import { useOnClickOutside } from '../../hooks/useOnClickOutside';
+import { usePublicSettings } from '../../hooks/usePublicSettings';
 import { PasswordChangeModal } from './PasswordChangeModal';
 import { LanguageSelector } from '../common';
 import { notificationsService } from '../../services/notifications.service';
 import { toast } from 'react-toastify';
-import { buildResourceUrl, getApiBaseUrl } from '../../utils/url';
+import { buildResourceUrl } from '../../utils/url';
 
 interface AdminHeaderProps {
   onMenuClick: () => void;
@@ -31,16 +32,7 @@ export const AdminHeader: React.FC<AdminHeaderProps> = ({ onMenuClick }) => {
   const [showPasswordModal, setShowPasswordModal] = useState(false);
   const queryClient = useQueryClient();
   
-  // Fetch branding settings
-  const { data: brandingSettings } = useQuery({
-    queryKey: ['admin-settings', 'branding'],
-    queryFn: async () => {
-      const response = await fetch(`${getApiBaseUrl()}/public/settings`);
-      if (response.ok) return response.json();
-      return null;
-    },
-    staleTime: 5 * 60 * 1000,
-  });
+  const { data: brandingSettings } = usePublicSettings();
 
   const companyName = brandingSettings?.branding_company_name?.trim() || 'PicPeak';
   const logoUrl = brandingSettings?.branding_logo_url?.trim();