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/master-classes/page.tsx

@@ -34,8 +34,6 @@ function PriceField({ label, value, onChange, placeholder }: { label: string; va
 
 interface MasterClassesData {
   title: string;
-  successMessage?: string;
-  waitingListText?: string;
   items: MasterClassItem[];
 }
 
@@ -386,21 +384,6 @@ export default function MasterClassesEditorPage() {
             onChange={(v) => update({ ...data, title: v })}
           />
 
-          <InputField
-            label="Текст после записи (success popup)"
-            value={data.successMessage || ""}
-            onChange={(v) => update({ ...data, successMessage: v || undefined })}
-            placeholder="Вы записаны! Мы свяжемся с вами"
-          />
-
-          <TextareaField
-            label="Текст для листа ожидания"
-            value={data.waitingListText || ""}
-            onChange={(v) => update({ ...data, waitingListText: v || undefined })}
-            placeholder="Все места заняты, но мы добавили вас в лист ожидания..."
-            rows={2}
-          />
-
           <ArrayEditor
             label="Мастер-классы"
             items={data.items}