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/features/admin/shell/admin-nav.model.ts

@@ -39,6 +39,7 @@ export const ADMIN_NAV_PRIMARY: AdminNavEntry[] = [
   { type: 'link', id: 'transactions', icon: 'creditCard', labelKey: 'adminShell.nav.transactions', path: ['transactions'] },
   { type: 'link', id: 'moderation', icon: 'star', labelKey: 'adminShell.nav.moderation', path: ['moderation'] },
   { type: 'link', id: 'notifications', icon: 'bell', labelKey: 'adminShell.nav.notifications', path: ['notifications'] },
+  { type: 'link', id: 'integrations', icon: 'network', labelKey: 'adminShell.nav.integrations', path: ['integrations'] },
   { type: 'link', id: 'reports', icon: 'chartBar', labelKey: 'adminShell.nav.reports', path: ['reports'] },
   { type: 'group', labelKey: 'adminShell.nav.partnersGroup' },
   { type: 'link', id: 'seller-management', icon: 'store', labelKey: 'adminShell.nav.sellerManagement', path: ['partners', 'seller-management'] },