Make video loading dynamic.

frontend/src/app/app.ts

@@ -28,10 +28,7 @@ export class App implements OnInit {
     loader: ({}) => firstValueFrom(this.#httpClient.get<LibraryItem[]>('/api/library'))
   });
 
-  firstVideo = computed(() => {
-    const libraryItems = this.libraryItems.value() ?? [];
-    return libraryItems.length ? libraryItems[0] : null
-  });
+  activeVideo = signal<LibraryItem | null>(null);
 
   ngOnInit(): void {
     this.loadLibrary();