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/features/admin/notifications/services/admin-notifications-gateway.token.ts

@@ -0,0 +1,9 @@
+import { InjectionToken, inject } from '@angular/core';
+import { AdminNotificationsGateway } from './admin-notifications-gateway.interface';
+import { AdminNotificationsLocalGateway } from './admin-notifications-local.gateway';
+
+/** Swap point for docs/backend/PHASE-2-ORDERS-NOTIFICATIONS-CONTRACT.md §6. */
+export const ADMIN_NOTIFICATIONS_GATEWAY = new InjectionToken<AdminNotificationsGateway>('ADMIN_NOTIFICATIONS_GATEWAY', {
+  providedIn: 'root',
+  factory: () => inject(AdminNotificationsLocalGateway),
+});