feat: centralize popup texts in new admin tab /admin/popups

- New admin page for shared popup texts (success, waiting list, error, Instagram hint)
- Removed popup fields from MC and Open Day admin editors
- All SignupModals now read from centralized popups config
- Stored as "popups" section in DB with fallback defaults

src/app/admin/open-day/page.tsx

@@ -19,8 +19,6 @@ interface OpenDayEvent {
   discountThreshold: number;
   minBookings: number;
   maxParticipants: number;
-  successMessage?: string;
-  waitingListText?: string;
   active: boolean;
 }
 
@@ -102,29 +100,6 @@ function EventSettings({
         />
       </div>
 
-      <div className="grid gap-4 sm:grid-cols-2">
-        <div>
-          <label className="block text-sm text-neutral-400 mb-1.5">Текст после записи</label>
-          <textarea
-            value={event.successMessage || ""}
-            onChange={(e) => onChange({ successMessage: e.target.value || undefined })}
-            rows={2}
-            className="w-full rounded-lg border border-white/10 bg-neutral-800 px-4 py-2.5 text-white placeholder-neutral-500 outline-none focus:border-gold transition-colors resize-none"
-            placeholder="Вы записаны!"
-          />
-        </div>
-        <div>
-          <label className="block text-sm text-neutral-400 mb-1.5">Текст для листа ожидания</label>
-          <textarea
-            value={event.waitingListText || ""}
-            onChange={(e) => onChange({ waitingListText: e.target.value || undefined })}
-            rows={2}
-            className="w-full rounded-lg border border-white/10 bg-neutral-800 px-4 py-2.5 text-white placeholder-neutral-500 outline-none focus:border-gold transition-colors resize-none"
-            placeholder="Все места заняты, но мы добавили вас в лист ожидания..."
-          />
-        </div>
-      </div>
-
       <div>
         <label className="block text-sm text-neutral-400 mb-1.5">Цена за занятие (BYN)</label>
         <input