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/en.ts

@@ -2060,6 +2060,7 @@ export const en: Translations = {
       transactions: 'Transactions',
       moderation: 'Reviews & Moderation',
       notifications: 'Notifications',
+      integrations: 'Integrations',
       reviews: 'Reviews',
       reports: 'Reports',
       partnersGroup: 'Partners',

src/app/i18n/hy.ts

@@ -2054,6 +2054,7 @@ export const hy: Translations = {
       customers: 'Հաճախորդներ',
       moderation: 'Կարծիքներ և մոդերացիա',
       notifications: 'Ծանուցումներ',
+      integrations: 'Ինտեգրումներ',
       transactions: 'Գործարքներ',
       reviews: 'Կարծիքներ',
       reports: 'Հաշվետվություններ',

src/app/i18n/ru.ts

@@ -2054,6 +2054,7 @@ export const ru: Translations = {
       customers: 'Клиенты',
       moderation: 'Отзывы и модерация',
       notifications: 'Уведомления',
+      integrations: 'Интеграции',
       transactions: 'Транзакции',
       reviews: 'Отзывы',
       reports: 'Отчёты',

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;