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/i18n/translations.ts

@@ -844,4 +844,17 @@ export interface Translations {
     optionalDataMissingDescription: string;
     optionalDataMissingResolution: string;
   };
+  mediaLibrary: {
+    title: string;
+    upload: string;
+    uploading: string;
+    searchPlaceholder: string;
+    delete: string;
+    deleteConfirmTitle: string;
+    deleteConfirmBody: string;
+    cancel: string;
+    confirm: string;
+    emptyTitle: string;
+    emptyDescription: string;
+  };
 }