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/components/sections/MasterClasses.tsx

@@ -11,6 +11,7 @@ import type { SiteContent, MasterClassItem, MasterClassSlot } from "@/types";
 interface MasterClassesProps {
   data: SiteContent["masterClasses"];
   regCounts?: Record<string, number>;
+  popups?: SiteContent["popups"];
 }
 
 const MONTHS_RU = [
@@ -217,7 +218,7 @@ function MasterClassCard({
   );
 }
 
-export function MasterClasses({ data, regCounts = {} }: MasterClassesProps) {
+export function MasterClasses({ data, regCounts = {}, popups }: MasterClassesProps) {
   const [signupTitle, setSignupTitle] = useState<string | null>(null);
 
   const upcoming = useMemo(() => {
@@ -278,8 +279,10 @@ export function MasterClasses({ data, regCounts = {} }: MasterClassesProps) {
         subtitle={signupTitle ?? ""}
         endpoint="/api/master-class-register"
         extraBody={{ masterClassTitle: signupTitle }}
-        successMessage={data.successMessage}
-        waitingMessage={data.waitingListText}
+        successMessage={popups?.successMessage}
+        waitingMessage={popups?.waitingListText}
+        errorMessage={popups?.errorMessage}
+        instagramHint={popups?.instagramHint}
       />
     </section>
   );