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/services/publicSettings.service.ts

@@ -12,6 +12,13 @@ export interface PublicSettings {
   branding_watermark_size: number;
   branding_favicon_url: string;
   branding_logo_url: string;
+  branding_logo_size?: string;
+  branding_logo_max_height?: number;
+  branding_logo_position?: 'left' | 'center' | 'right';
+  branding_logo_display_mode?: 'logo_only' | 'text_only' | 'logo_and_text';
+  branding_logo_display_header?: boolean;
+  branding_logo_display_hero?: boolean;
+  branding_hide_powered_by?: boolean;
   theme_config: any;
   default_language: string;
   enable_analytics: boolean;
@@ -34,6 +41,10 @@ export interface PublicSettings {
   event_default_require_password?: boolean;
   gallery_show_filter_bar?: boolean;
   event_phone_field_enabled?: boolean;
+  // SEO meta tags (consumed by RobotsMetaTags)
+  seo_meta_noindex?: boolean;
+  seo_meta_nofollow?: boolean;
+  seo_meta_noai?: boolean;
 }
 
 export const publicSettingsService = {