fix(backoffice): add error+retry states to Users, Monitoring, Analytics, Reports

Phase 8 (RC-01): these 4 list/dashboard pages had no error-state handling
on their primary data-load subscriptions — on a gateway error, `loading`
was either never reset (Users, Monitoring, Analytics: genuine infinite-
spinner risk, nested subscribe chain in Analytics never resolved on
failure) or there was no loading/empty/error handling at all (Reports
queue: raw table with zero skeleton or fallback).

- admin-users.facade.ts, admin-monitoring.facade.ts: add `error` signal,
  error callback on the primary load subscribe so `loading` always
  resolves.
- admin-analytics.facade.ts: add `error` signal; every level of the
  4-deep nested gateway subscribe chain (orders -> products ->
  categories -> reviews) now has an error handler that resolves loading
  instead of leaving it stuck true.
- admin-moderation.facade.ts: add `reportsLoading`/`reportsError` signals
  (reports list had none previously).
- Templates: reuse existing `app-skeleton`/`app-empty-state`/`app-button`
  primitives for the new error branch, `common.retry` label, two new
  generic `common.errorTitle`/`common.errorDescription` i18n keys added
  to en/ru/hy (reused across all 4 fixes instead of one-off per-page
  copy).

Verified: tsc --noEmit clean, `npm run build` green (pre-existing bundle-
budget warning only, unrelated). Live-checked Home (375px) and Backoffice
Products (1024px) — no console errors, tables/cards render without
overflow.

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

src/app/features/admin/users/pages/admin-users-page.component.html

@@ -6,6 +6,12 @@
         @for (i of [1,2,3]; track i) { <app-skeleton shape="rect" height="40px" /> }
         <span class="sr-only">{{ 'common.loading' | translate }}</span>
       </div>
+    } @else if (facade.error()) {
+      <app-empty-state [title]="'common.errorTitle' | translate" [description]="'common.errorDescription' | translate">
+        <span slot="actions">
+          <app-button variant="primary" (click)="facade.loadAll()">{{ 'common.retry' | translate }}</app-button>
+        </span>
+      </app-empty-state>
     } @else if (facade.users().length === 0) {
       <app-empty-state [title]="'adminUsers.emptyTitle' | translate" [description]="'adminUsers.emptyDescription' | translate" />
     } @else {