feat(media): add Media Manager UI (grid, upload, delete)

MediaLibraryPageComponent replaces the coming-soon placeholder at
/backoffice/media. Built entirely on Sprint 6 Design System primitives
(app-card, app-button, app-input, app-empty-state, app-dialog, app-pagination,
app-skeleton) and Sprint 4 Task 2's MediaRepository/MockMediaRepository.

- Grid view with per-tile filename/size and delete action
- Hidden native file input triggered by an app-button, uploads via
  MediaLibraryFacade -> MediaRepository.upload()
- Delete requires confirmation through app-dialog (destructive action)
- Search + pagination wired to MockMediaRepository's list() params
- Loading state shows app-skeleton tiles; empty state shows app-empty-state
- New mediaLibrary.* translation namespace across en/ru/hy

Verified in browser (via a temporary unguarded route, reverted before
commit - /backoffice/media itself requires Telegram QR admin auth not
available in this session): empty state renders correctly with translated
copy, search input and upload button present, no console errors.

src/app/app.routes.ts

@@ -93,7 +93,7 @@ const coreRoutes: Routes = [
       },
       {
         path: 'media',
-        loadComponent: () => import('./features/backoffice/shared/backoffice-coming-soon-page.component').then(m => m.BackofficeComingSoonPageComponent),
+        loadComponent: () => import('./features/backoffice/media/media-library-page.component').then(m => m.MediaLibraryPageComponent),
         data: { titleKey: 'dashboard.actionMediaLibrary' }
       },
       { path: '**', redirectTo: 'dashboard' }