Add IMDb movie importer feature and unify header

Introduces a new movie importer feature allowing admins to search and import movies from IMDb, including new components for search, result display, and error handling. Replaces the schedule header with a reusable menu header component. Updates routing, navigation, and HTTP service to support the new importer. Adds a poster placeholder image and improves poster error handling.

src/app/movie-poster/movie-poster.component.ts

@@ -9,4 +9,13 @@ import { Film } from '@infinimotion/model-frontend';
 })
 export class MoviePosterComponent {
   movie = input.required<Film>();
+
+  onPosterError(event: Event) {
+    const img = event.target as HTMLImageElement;
+    const placeholder = 'assets/poster_placeholder.png';
+
+    if (img.src !== window.location.origin + placeholder) {
+      img.src = placeholder;
+    }
+  }
 }