feat: blur booking codes setting + two-column settings page — closes #114

- New display setting "Blur Booking Codes" (off by default)
- When enabled, confirmation codes are blurred across all views
  (ReservationsPanel, DayDetailPanel, Transport detail modal)
- Hover or click reveals the code (click toggles on mobile)
- Settings page uses masonry two-column layout on desktop, single
  column on mobile (<900px)
- Fix hardcoded admin page title to use i18n key

client/src/pages/AdminPage.tsx

@@ -333,7 +333,7 @@ export default function AdminPage(): React.ReactElement {
               <Shield className="w-5 h-5 text-slate-700" />
             </div>
             <div>
-              <h1 className="text-2xl font-bold text-slate-900">Administration</h1>
+              <h1 className="text-2xl font-bold text-slate-900">{t('admin.title')}</h1>
               <p className="text-slate-500 text-sm">{t('admin.subtitle')}</p>
             </div>
           </div>