feat(admin): build shared admin shell

Sidebar (Dashboard/Catalog group/Products/Categories/Orders/Transactions/
Reviews/Reports/Content/Media/Marketplace Builder/Users/Settings/
Monitoring/Analytics + Documentation/Help/Logout), sticky topbar
(breadcrumbs, page title/description, search, notifications, tenant
selector and quick-publish placeholders, current user), reserved
right-rail slot, scrollable content area. Desktop 280px sidebar, tablet
icon rail, mobile drawer with focus management and Escape-to-close.

Nav items without a built page (Reviews, Reports, Settings, Docs, Help)
render disabled with a coming-soon badge instead of dead links; Content
and Marketplace Builder route to the existing project-editor pages
(static-pages / general) rather than duplicating them.

All 15 /backoffice/** routes now render through AdminLayoutComponent;
the public storefront header/back-button/footer no longer render on
admin routes (app.ts/app.html gate on a new isAdminRoute signal).

Added the adminShell i18n namespace (ru/en/hy) for every new shell
string so this doesn't add to the existing untranslated-admin-UI gap
tracked in KNOWN-ISSUES.md.

Colors/type sizes follow DESIGN.md tokens; the two rgba() modal-scrim
values are a documented, intentional exception (neutral overlay,
not a themed token).

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

src/app/app.ts

@@ -28,6 +28,7 @@ import { TelegramLoginComponent } from './components/telegram-login/telegram-log
 export class App implements OnInit {
   protected title = '';
   isHomePage = signal(true);
+  isAdminRoute = signal(false);
   checkingServer = signal(true);
   serverAvailable = signal(false);
 
@@ -62,6 +63,9 @@ export class App implements OnInit {
         const url = navEnd.urlAfterRedirects || navEnd.url;
         // Home pages: /ru, /en, /hy (with or without trailing slash)
         this.isHomePage.set(/^\/[a-z]{2}\/?$/.test(url) || url === '/' || url === '');
+        // Admin backoffice owns its own shell (AdminLayoutComponent) - the
+        // storefront header/back-button/footer never render there.
+        this.isAdminRoute.set(/^\/[a-z]{2}\/backoffice(\/|$|\?)/.test(url));
       });
   }