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/features/backoffice/media/facade/media-library.facade.ts

@@ -0,0 +1,71 @@
+import { Injectable, inject, signal } from '@angular/core';
+import { MediaRepository } from '../../../../core/media/media-repository';
+import { MediaAsset } from '../../../../core/media/models/media-asset.model';
+
+const PAGE_SIZE = 24;
+
+@Injectable({ providedIn: 'root' })
+export class MediaLibraryFacade {
+  private readonly repository = inject(MediaRepository);
+
+  readonly items = signal<MediaAsset[]>([]);
+  readonly total = signal(0);
+  readonly loading = signal(false);
+  readonly uploading = signal(false);
+  readonly error = signal<string | null>(null);
+  readonly search = signal('');
+  readonly page = signal(1);
+
+  async load(): Promise<void> {
+    this.loading.set(true);
+    this.error.set(null);
+    try {
+      const result = await this.repository.list({
+        page: this.page(),
+        pageSize: PAGE_SIZE,
+        search: this.search(),
+      });
+      this.items.set(result.items);
+      this.total.set(result.total);
+    } catch {
+      this.error.set('Failed to load media assets.');
+    } finally {
+      this.loading.set(false);
+    }
+  }
+
+  setSearch(value: string): void {
+    this.search.set(value);
+    this.page.set(1);
+    void this.load();
+  }
+
+  setPage(page: number): void {
+    this.page.set(page);
+    void this.load();
+  }
+
+  async upload(file: File): Promise<void> {
+    this.uploading.set(true);
+    this.error.set(null);
+    try {
+      await this.repository.upload(file);
+      this.page.set(1);
+      await this.load();
+    } catch {
+      this.error.set('Failed to upload file.');
+    } finally {
+      this.uploading.set(false);
+    }
+  }
+
+  async remove(id: string): Promise<void> {
+    this.error.set(null);
+    try {
+      await this.repository.remove(id);
+      await this.load();
+    } catch {
+      this.error.set('Failed to delete file.');
+    }
+  }
+}