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/contexts/MaintenanceContext.tsx

@@ -1,7 +1,6 @@
 import React, { createContext, useContext, useState, useEffect, ReactNode } from 'react';
-import { useQuery } from '@tanstack/react-query';
 import { setMaintenanceModeCallback } from '../config/api';
-import { getApiBaseUrl } from '../utils/url';
+import { usePublicSettings } from '../hooks/usePublicSettings';
 
 interface MaintenanceContextType {
   isMaintenanceMode: boolean;
@@ -25,34 +24,18 @@ interface MaintenanceProviderProps {
 export const MaintenanceProvider: React.FC<MaintenanceProviderProps> = ({ children }) => {
   const [isMaintenanceMode, setIsMaintenanceMode] = useState(false);
 
-  // Check maintenance mode status on mount
-  const { data: settings } = useQuery({
-    queryKey: ['public-settings-maintenance'],
-    queryFn: async () => {
-      try {
-        const response = await fetch(`${getApiBaseUrl()}/public/settings`);
-        if (response.status === 503) {
-          setIsMaintenanceMode(true);
-          return null;
-        }
-        return response.json();
-      } catch (error) {
-        // If we can't reach the server, don't assume maintenance mode
-        return null;
-      }
-    },
-    staleTime: 30 * 1000, // Check every 30 seconds
-    refetchInterval: 30 * 1000,
-  });
+  // Polls /public/settings every 30s so a maintenance flag flipped server-side propagates
+  // without a refresh. 503 responses are caught by the axios interceptor in config/api.ts
+  // (which calls setMaintenanceModeCallback below), so we only need to read the explicit
+  // maintenance_mode flag here.
+  const { data: settings } = usePublicSettings({ refetchInterval: 30_000 });
 
-  // Update maintenance mode based on settings
   useEffect(() => {
     if (settings?.maintenance_mode !== undefined) {
       setIsMaintenanceMode(settings.maintenance_mode);
     }
   }, [settings]);
 
-  // Set up the callback for API interceptor
   useEffect(() => {
     setMaintenanceModeCallback((enabled: boolean) => {
       setIsMaintenanceMode(enabled);