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/components/ThreadComposer.vue

@@ -29,7 +29,8 @@
 import { computed } from "vue";
 import type { UploadUserFile } from "element-plus";
 import { displayDateTime, displayUser } from "../utils/display";
-import { clientRuntime, pickFiles } from "../runtime";
+import { clientRuntime } from "../runtime";
+import { pickFilesWithFeedback } from "../utils/fileTaskFeedback";
 import { TEXT } from "../locales";
 
 const props = defineProps<{
@@ -65,7 +66,7 @@ const fileListProxy = computed({
 const quoteContent = (item: any) => (item?.is_deleted ? TEXT.modules.knowledgeMedicalConsult.quoteDeleted : item?.content || TEXT.common.fallback);
 
 const pickNativeAttachments = async () => {
-  const files = await pickFiles({ multiple: true, title: TEXT.common.labels.attachments });
+  const files = await pickFilesWithFeedback({ multiple: true, title: TEXT.common.labels.attachments });
   const existing = new Set(props.fileList.map((item) => `${item.name}:${item.size}`));
   const additions: UploadUserFile[] = files
     .filter((file) => !existing.has(`${file.name}:${file.size}`))