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.interface.ts

@@ -0,0 +1,8 @@
+import { Observable } from 'rxjs';
+import { AdminNotification, AdminNotificationFilters } from '../models/admin-notification.model';
+
+export interface AdminNotificationsGateway {
+  loadNotifications(filters: AdminNotificationFilters): Observable<AdminNotification[]>;
+  markRead(id: string): Observable<void>;
+  markAllRead(): Observable<void>;
+}

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),
+});

src/app/features/admin/notifications/services/admin-notifications-local.gateway.ts

@@ -0,0 +1,50 @@
+import { Injectable, inject } from '@angular/core';
+import { Observable, map, of } from 'rxjs';
+import { AdminNotification, AdminNotificationFilters } from '../models/admin-notification.model';
+import { AdminNotificationsGateway } from './admin-notifications-gateway.interface';
+import { ADMIN_ORDERS_GATEWAY } from '../../orders/services/admin-orders-gateway.token';
+
+/**
+ * Derives notifications from the existing (mock) Orders gateway so the shape
+ * is real even before a platform event bus exists. Once
+ * docs/backend/PHASE-2-ORDERS-NOTIFICATIONS-CONTRACT.md §6 ships, swap the
+ * token - no caller changes needed.
+ */
+@Injectable({ providedIn: 'root' })
+export class AdminNotificationsLocalGateway implements AdminNotificationsGateway {
+  private readonly ordersGateway = inject(ADMIN_ORDERS_GATEWAY);
+  private readonly readIds = new Set<string>();
+
+  loadNotifications(filters: AdminNotificationFilters): Observable<AdminNotification[]> {
+    return this.ordersGateway.loadOrders({ search: '', status: 'all', page: 1, pageSize: 20 }).pipe(
+      map(result => result.items
+        .map((order): AdminNotification => ({
+          id: `notif_${order.id}`,
+          marketplaceId: 'default',
+          entityType: 'order',
+          entityId: order.id,
+          severity: order.status === 'cancelled' ? 'warning' : 'info',
+          eventType: order.payment.status === 'paid' ? 'order.paid' : 'order.created',
+          title: `Order ${order.orderNumber} - ${order.payment.status === 'paid' ? 'paid' : 'created'}`,
+          read: this.readIds.has(order.id),
+          deepLink: ['orders', order.id],
+          createdAt: order.createdAt,
+        }))
+        .filter(n => !filters.unreadOnly || !n.read)
+        .filter(n => !filters.eventType || filters.eventType === 'all' || n.eventType === filters.eventType)
+      )
+    );
+  }
+
+  markRead(id: string): Observable<void> {
+    const orderId = id.replace(/^notif_/, '');
+    this.readIds.add(orderId);
+    return of(void 0);
+  }
+
+  markAllRead(): Observable<void> {
+    return this.loadNotifications({ marketplaceId: '', eventType: '', unreadOnly: false }).pipe(
+      map(items => { items.forEach(n => this.readIds.add(n.entityId)); })
+    );
+  }
+}