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/facade/admin-notifications.facade.ts

@@ -0,0 +1,36 @@
+import { Injectable, computed, inject, signal } from '@angular/core';
+import { take } from 'rxjs/operators';
+import { AdminNotification, AdminNotificationFilters } from '../models/admin-notification.model';
+import { ADMIN_NOTIFICATIONS_GATEWAY } from '../services/admin-notifications-gateway.token';
+
+@Injectable({ providedIn: 'root' })
+export class AdminNotificationsFacade {
+  private readonly gateway = inject(ADMIN_NOTIFICATIONS_GATEWAY);
+
+  readonly filters = signal<AdminNotificationFilters>({ marketplaceId: '', eventType: 'all', unreadOnly: false });
+  readonly notifications = signal<AdminNotification[]>([]);
+  readonly loading = signal(false);
+
+  readonly unreadCount = computed(() => this.notifications().filter(n => !n.read).length);
+
+  load(): void {
+    this.loading.set(true);
+    this.gateway.loadNotifications(this.filters()).pipe(take(1)).subscribe(items => {
+      this.notifications.set(items);
+      this.loading.set(false);
+    });
+  }
+
+  updateFilters(patch: Partial<AdminNotificationFilters>): void {
+    this.filters.update(current => ({ ...current, ...patch }));
+    this.load();
+  }
+
+  markRead(id: string): void {
+    this.gateway.markRead(id).pipe(take(1)).subscribe(() => this.load());
+  }
+
+  markAllRead(): void {
+    this.gateway.markAllRead().pipe(take(1)).subscribe(() => this.load());
+  }
+}