feat: Phase 4 frontend - Integrations backoffice section (mock-gateway backed)

New core/integrations (Connector/DeadLetterEntry models + gateway/token)
and features/admin/integrations (facade + page: connector table with
status/lag/errors/backlog/unmatched, pause/resume). Seeded empty per
Sprint 0.1's "no fixed partner list" decision - the section is ready to
populate the moment the first real connector is onboarded against
docs/backend/PHASE-4-CONNECTOR-FRAMEWORK-CONTRACT.md §7. New /backoffice/
integrations route + nav entry, nav i18n key in all 3 languages.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/i18n/translations.ts

@@ -2069,6 +2069,7 @@ export interface Translations {
       reviews: string;
       moderation: string;
       notifications: string;
+      integrations: string;
       reports: string;
       partnersGroup: string;
       sellerManagement: string;