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/shell/admin-nav.model.ts

@@ -38,6 +38,7 @@ export const ADMIN_NAV_PRIMARY: AdminNavEntry[] = [
   { type: 'link', id: 'customers', icon: 'user', labelKey: 'adminShell.nav.customers', path: ['customers'] },
   { type: 'link', id: 'transactions', icon: 'creditCard', labelKey: 'adminShell.nav.transactions', path: ['transactions'] },
   { type: 'link', id: 'moderation', icon: 'star', labelKey: 'adminShell.nav.moderation', path: ['moderation'] },
+  { type: 'link', id: 'notifications', icon: 'bell', labelKey: 'adminShell.nav.notifications', path: ['notifications'] },
   { type: 'link', id: 'reports', icon: 'chartBar', labelKey: 'adminShell.nav.reports', path: ['reports'] },
   { type: 'group', labelKey: 'adminShell.nav.partnersGroup' },
   { type: 'link', id: 'seller-management', icon: 'store', labelKey: 'adminShell.nav.sellerManagement', path: ['partners', 'seller-management'] },