refactor(install): 重构下载安装队列逻辑，拆分并发控制

1.  将下载与安装流程分离，实现并发下载限制与单安装实例控制
2.  新增任务阶段枚举，替换原有activeDownloads计数逻辑
3.  优化取消任务的处理流程，区分运行中和排队中的任务
4.  修复下载队列显示的统计错误，改为展示已完成任务数
5.  调整日志格式与代码注释，提升可读性

src/components/DownloadQueue.vue

@@ -15,7 +15,7 @@
           v-if="downloads.length"
           class="rounded-full bg-slate-100 px-2 py-0.5 text-xs font-medium text-slate-500 dark:bg-slate-800/70 dark:text-slate-300"
         >
-          ({{ activeDownloads }}/{{ downloads.length }})
+          ({{ completedDownloads }}/{{ downloads.length }})
         </span>
       </div>
       <div class="flex items-center gap-2">
@@ -148,10 +148,8 @@ const emit = defineEmits<{
 
 const isExpanded = ref(false);
 
-const activeDownloads = computed(() => {
-  return props.downloads.filter(
-    (d) => d.status === "downloading" || d.status === "installing",
-  ).length;
+const completedDownloads = computed(() => {
+  return props.downloads.filter((d) => d.status === "completed").length;
 });
 
 const toggleExpand = () => {