feat(desktop): 稳定桌面端界面与文件操作反馈

- 重构 DesktopPreferences 为分栏式设置面板，整合连接、外观、通知、更新与诊断信息分区，并补充过渡动效与暗色主题样式
- DesktopLayout 侧边栏导航分组支持展开折叠，调整管理/项目区块顺序并统一图标与标题
- 新增 fileTaskFeedback 工具，统一 pickFiles/saveFile/openFile 的成功/取消提示，替换审计导出、权限日志、附件、文档、线程、项目配置等处的直接调用
- desktopUpdateManager 暴露更新状态快照与状态变更监听，区分检查中、安装中、已推迟、失败等状态
- DesktopServerSettings 增加连接诊断信息（检查时间、健康地址、耗时、HTTP 状态）
- unified-page.css 与 ProjectMilestones 引入 CSS 变量以适配暗色主题
- WebLayout 将服务器设置入口改为打开系统偏好面板，管理菜单中邮件服务归入系统设置分组
- ProfileSettings 移除已迁入偏好面板的桌面端专属区块
- 补充 Layout.desktop 布局与偏好面板契约测试

frontend/src/views/ia/RiskIssueMonitoringVisits.vue

@@ -466,7 +466,7 @@ import {
 import { fetchSites } from "../../api/sites";
 import StateEmpty from "../../components/StateEmpty.vue";
 import { TEXT } from "../../locales";
-import { pickFiles, saveFile } from "../../runtime";
+import { pickFilesWithFeedback, saveFileWithFeedback } from "../../utils/fileTaskFeedback";
 import { useAuthStore } from "../../store/auth";
 import { useStudyStore } from "../../store/study";
 import type { Site } from "../../types/api";
@@ -978,7 +978,7 @@ const handleExportExcel = async () => {
     const contentType = response.headers?.["content-type"] || "application/octet-stream";
     const filename = getFilename(response.headers?.["content-disposition"]) || "监查访视问题.xlsx";
     const blob = new Blob([response.data], { type: contentType });
-    await saveFile({ suggestedName: filename, mimeType: contentType, data: blob });
+    await saveFileWithFeedback({ suggestedName: filename, mimeType: contentType, data: blob });
     ElMessage.success("导出成功");
   } catch (e: any) {
     ElMessage.error(e?.response?.data?.detail || TEXT.common.messages.downloadFailed);
@@ -1010,7 +1010,7 @@ const importIssueFile = async (file: File) => {
 };
 
 const selectImportFile = async () => {
-  const [file] = await pickFiles({
+  const [file] = await pickFilesWithFeedback({
     multiple: false,
     accept: ["xlsx", "csv"],
     title: "导入监查访视问题",