refactor(course-overview): simplify activeRepository getter

Replace complex filtering chain with direct sorting of userRepositories.
This improves code readability and performance by avoiding multiple
filterBy calls and creating fewer intermediate arrays. The change keeps
the logic to return the most recent repository based on lastSubmissionAt.

Author: rohitpaulk

app/controllers/course-overview.ts

@@ -10,18 +10,7 @@ export default class CourseOverviewController extends Controller {
   @service declare authenticator: AuthenticatorService;
 
   get activeRepository(): RepositoryModel | null {
-    if (this.authenticator.currentUser) {
-      return (
-        this.authenticator.currentUser.repositories
-          .filterBy('course', this.model.course)
-          .filterBy('firstSubmissionCreated')
-          .sortBy('lastSubmissionAt')
-          .toArray()
-          .reverse()[0] || null
-      );
-    } else {
-      return null;
-    }
+    return this.userRepositories.sortBy('lastSubmissionAt').toArray().reverse()[0] || null;
   }
 
   get currentUser() {