Replace duplicate assignments link with missing "sign up"

Anonymous users had two links to assignments in navigation, but none for signing up.

app/views/layouts/application.html.erb

@@ -109,8 +109,8 @@
           <% else %>
             <!-- Not logged in state -->
             <div class="flex items-center space-x-4">
-              <%= link_to "Assignments", assignments_path, class: "text-gray-600 hover:text-gray-900 dark:text-gray-300 dark:hover:text-white" %>
               <%= link_to "Log in", new_user_session_path, class: "text-gray-600 hover:text-gray-900 dark:text-gray-300 dark:hover:text-white" %>
+              <%= link_to "Sign up", new_user_registration_path, class: "text-gray-600 hover:text-gray-900 dark:text-gray-300 dark:hover:text-white" %>
             </div>
 
             <!-- Dark mode toggle button -->