fix(ui): 视频合成页 BGM 选择后支持清除

tauri-app/src/pages/VideoCreation/VideoCompose.tsx

@@ -418,9 +418,21 @@ export default function VideoCompose() {
                     </div>
                   </div>
                 </div>
-                <button className="btn btn-ghost btn-sm" onClick={e => { e.stopPropagation(); setBgmModalOpen(true); }}>
-                  更换
-                </button>
+                <div style={{ display: 'flex', gap: 8 }}>
+                  <button className="btn btn-ghost btn-sm" onClick={e => { e.stopPropagation(); setBgmModalOpen(true); }}>
+                    更换
+                  </button>
+                  <button
+                    className="btn btn-ghost btn-sm"
+                    style={{ color: 'var(--text-secondary)' }}
+                    onClick={e => {
+                      e.stopPropagation();
+                      setBgmMusic(undefined, undefined, undefined);
+                    }}
+                  >
+                    清除
+                  </button>
+                </div>
               </div>
             ) : (
               <div