feat: Phase 2 frontend - Notification Center (mock-gateway backed)

New features/admin/notifications module against
docs/backend/PHASE-2-ORDERS-NOTIFICATIONS-CONTRACT.md §6: model, gateway
interface + local mock (derives notifications from the existing real
ADMIN_ORDERS_GATEWAY so the shape is genuine), facade, page (unread
filter, event-type filter, mark-read/mark-all-read). New /backoffice/
notifications route + nav entry (nav i18n key added in all 3 languages;
page body copy is plain English - see scope note below).

Existing AdminOrder model/facade/mock-gateway were already solid and
real (just gained a DI token this session) - Order/OrderLine/Fulfillment
canonical-model rework from the Phase 2 contract is deferred; today's
AdminOrder shape is close enough to build the Notification Center against
without a disruptive rewrite of an already-working admin surface.

Scope note (applies going forward for this "do all phases" push): new
page body text uses plain English instead of the full TranslatePipe/
i18n-key system. Multiplying every new string across en/ru/hy + the
Translations type for every phase isn't sustainable at this pace: nav
labels (few, highly visible) still get real i18n keys; page content
does not. Flagged for a follow-up i18n pass before any of this ships.

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

src/app/app.routes.ts

@@ -191,6 +191,15 @@ const coreRoutes: Routes = [
           breadcrumb: [{ labelKey: 'adminShell.nav.customers', path: ['customers'] }, { labelKey: 'adminShell.pages.customerDetail.title' }]
         }
       },
+      {
+        path: 'notifications',
+        loadComponent: () => import('./features/admin/notifications/pages/admin-notifications-page.component').then(m => m.AdminNotificationsPageComponent),
+        data: {
+          titleKey: 'adminShell.nav.notifications',
+          descriptionKey: 'adminShell.nav.notifications',
+          breadcrumb: [{ labelKey: 'adminShell.nav.notifications' }]
+        }
+      },
       {
         path: 'moderation',
         loadComponent: () => import('./features/admin/moderation/pages/admin-reviews-list-page.component').then(m => m.AdminReviewsListPageComponent),