fix: 重复 URL 添加时显示跳过提示而非误报入队成功

ui/src/app/app.component.ts

@@ -456,6 +456,11 @@ export class AppComponent implements OnInit, OnDestroy {
       this.addInProgress = false;
       if (status.status === 'error') {
         this.toastService.show(`添加失败: ${status.msg}`, { error: true });
+      } else if (status.msg) {
+        // Backend skipped the URL (e.g. "Download already exists") — surface
+        // that instead of a misleading "added to queue".
+        this.toastService.show(status.msg, { error: true });
+        this.loadEvents();
       } else {
         this.addUrl = '';
         this.toastService.show('已加入队列');