feat(admin): sprint 19 admin dashboard, routing, i18n

- Add admin dashboard feature (models/gateway/facade/components/page)
- Wire admin/products routes and backoffice coming-soon placeholders
- Add lastPublishedAt to ProjectEditorFacade/state
- Add dashboard i18n keys (en/ru/hy) and docs/ADMIN.md

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/features/backoffice/shared/backoffice-coming-soon-page.component.html

@@ -0,0 +1,5 @@
+<div class="coming-soon">
+  <h1 class="coming-soon__title">{{ titleKey | translate }}</h1>
+  <p class="coming-soon__description">{{ 'dashboard.comingSoonDescription' | translate }}</p>
+  <a class="coming-soon__link" [routerLink]="['../dashboard']">{{ 'dashboard.backToDashboard' | translate }}</a>
+</div>