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/admin/dashboard/models/admin-dashboard.model.ts

@@ -0,0 +1,42 @@
+export type AdminDashboardCardStatus = 'loading' | 'ready' | 'empty' | 'error' | 'pending-backend';
+
+export interface AdminDashboardCardState<T> {
+  status: AdminDashboardCardStatus;
+  value: T | null;
+}
+
+export interface AdminDashboardMetrics {
+  categoriesCount: number;
+  productsCount: number;
+}
+
+export type AdminDashboardQuickActionId =
+  | 'edit-project'
+  | 'categories'
+  | 'products'
+  | 'static-pages'
+  | 'transactions'
+  | 'orders'
+  | 'media-library'
+  | 'preview-marketplace';
+
+export interface AdminDashboardQuickAction {
+  id: AdminDashboardQuickActionId;
+  labelKey: string;
+  route: string[];
+  external?: boolean;
+}
+
+export type AdminDashboardActivityType = 'draft-saved' | 'published';
+
+export interface AdminDashboardActivityEntry {
+  id: string;
+  type: AdminDashboardActivityType;
+  timestamp: number;
+}
+
+export interface AdminDashboardHealthCheck {
+  code: string;
+  labelKey: string;
+  healthy: boolean;
+}