Fix mobile responsiveness across all pages

- BookingsPage: hide Номер/Выезд/Источник columns on small screens, show nights inline on mobile
- GuestsPage: stats grid 2→4 cols (grid-cols-2 lg:grid-cols-4), table columns hidden on mobile (md/lg breakpoints), GuestModal stats 2→4 cols on sm+
- BookingDetailPanel: full-width on mobile (w-full sm:w-[400px])
- Topbar: notifications panel full-width on mobile (w-[calc(100vw-1rem)] sm:w-96)
- LoyaltyPage, MaintenancePage, DiscountsPage, TariffsPage: stats grids grid-cols-2 lg:grid-cols-4

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/DiscountsPage.tsx

@@ -301,7 +301,7 @@ export function DiscountsPage() {
       </div>
 
       {/* Stats */}
-      <div className="grid grid-cols-4 gap-4">
+      <div className="grid grid-cols-2 lg:grid-cols-4 gap-3">
         {[
           { label: 'Всего скидок',  value: discounts.length },
           { label: 'Активных',      value: discounts.filter(d => d.isActive).length },