feat: add discount UI for landing pages

Add discount configuration in admin editor and countdown timer
display on public purchase page.

- Add discount section to AdminLandingEditor with toggle, percent
  slider, datetime pickers, per-tariff overrides, live preview
- Add DiscountBanner with countdown timer to QuickPurchase page
  using AnimatePresence exit animation and proper interval cleanup
- Add strikethrough prices and discount badges to tariff cards,
  summary card and pay button with null/equality guards
- Add discount translations for ru and en locales
- Fix delete/toggle API return types to match backend contracts
- Fix isoToDatetimeLocal to preserve seconds on re-edit
- Fix preview rounding to match backend integer division

src/pages/AdminLandings.tsx

@@ -160,6 +160,11 @@ function SortableLandingCard({
                     <GiftIcon />
                   </span>
                 )}
+                {landing.has_active_discount && (
+                  <span className="rounded-full bg-accent-500/20 px-2 py-0.5 text-[10px] font-medium text-accent-400">
+                    {t('admin.landings.discountActive', 'Discount')}
+                  </span>
+                )}
               </div>
               <div className="text-sm text-dark-400">
                 <span>