Ensure "Choose Music" Dropdown is always visible

The "Choose Music" dropdown at the top of the interface is hidden when only one piece is assigned and only reappears when a second piece is added. This prevents teachers from accessing the grading interface if only a single piece is assigned. Update the interface logic to ensure that the "Choose Music" dropdown is always visible, regardless of how many pieces are assigned.

首先跟踪控制 "Choose Music" 下拉菜单的界面逻辑，并使用一首已分配的曲目复现该行为。验证在分配了一首或多首曲目时下拉菜单仍保持可见，并且教师可以访问评分界面。