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/i18n/en.ts

@@ -2059,6 +2059,7 @@ export const en: Translations = {
       customers: 'Customers',
       transactions: 'Transactions',
       moderation: 'Reviews & Moderation',
+      notifications: 'Notifications',
       reviews: 'Reviews',
       reports: 'Reports',
       partnersGroup: 'Partners',

src/app/i18n/hy.ts

@@ -2053,6 +2053,7 @@ export const hy: Translations = {
       orders: 'Պատվերներ',
       customers: 'Հաճախորդներ',
       moderation: 'Կարծիքներ և մոդերացիա',
+      notifications: 'Ծանուցումներ',
       transactions: 'Գործարքներ',
       reviews: 'Կարծիքներ',
       reports: 'Հաշվետվություններ',

src/app/i18n/ru.ts

@@ -2053,6 +2053,7 @@ export const ru: Translations = {
       orders: 'Заказы',
       customers: 'Клиенты',
       moderation: 'Отзывы и модерация',
+      notifications: 'Уведомления',
       transactions: 'Транзакции',
       reviews: 'Отзывы',
       reports: 'Отчёты',

src/app/i18n/translations.ts

@@ -2068,6 +2068,7 @@ export interface Translations {
       transactions: string;
       reviews: string;
       moderation: string;
+      notifications: string;
       reports: string;
       partnersGroup: string;
       sellerManagement: string;