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/en.ts

@@ -846,4 +846,17 @@ export const en: Translations = {
     optionalDataMissingDescription: 'Optional configuration data is absent.',
     optionalDataMissingResolution: 'Add optional value if UX depends on it.',
   },
+  mediaLibrary: {
+    title: 'Media Library',
+    upload: 'Upload',
+    uploading: 'Uploading...',
+    searchPlaceholder: 'Search files...',
+    delete: 'Delete',
+    deleteConfirmTitle: 'Delete file?',
+    deleteConfirmBody: 'This cannot be undone.',
+    cancel: 'Cancel',
+    confirm: 'Delete',
+    emptyTitle: 'No files yet',
+    emptyDescription: 'Upload an image or document to get started.',
+  },
 };