功能(文档与桌面)：完善文件预览下载与客户端构建基线

- 保存文档版本原始文件名，规范下载响应并持久化上传目录\n- 增加 PDF.js 预览、桌面保存打开流程及统一错误反馈\n- 统一 Node.js 22.13 构建基线并收紧临时文件权限门禁\n- 补充迁移、单元测试、发布检查与运维文档

frontend/src/styles/main.css

@@ -1038,7 +1038,7 @@ body.is-desktop-runtime .el-overlay-message-box {
   overflow: auto;
 }
 
-body.is-desktop-runtime .el-dialog,
+body.is-desktop-runtime :where(.el-dialog),
 body.is-desktop-runtime .el-message-box {
   overflow: hidden;
   margin: 0 auto !important;
@@ -1052,7 +1052,7 @@ body.is-desktop-runtime .el-message-box {
   color: #0f172a;
 }
 
-body.is-desktop-runtime .el-dialog {
+body.is-desktop-runtime :where(.el-dialog) {
   max-width: min(calc(100vw - 72px), var(--el-dialog-width, 960px));
 }
 
@@ -1061,7 +1061,8 @@ body.is-desktop-runtime .profile-settings-dialog,
 body.is-desktop-runtime .desktop-preferences-dialog,
 body.is-desktop-runtime .user-form-dialog,
 body.is-desktop-runtime .reset-password-dialog,
-body.is-desktop-runtime .project-form-dialog {
+body.is-desktop-runtime .project-form-dialog,
+body.is-desktop-runtime .pdf-preview-dialog--flush {
   overflow: visible !important;
   border: none !important;
   border-radius: 0 !important;
@@ -1070,6 +1071,26 @@ body.is-desktop-runtime .project-form-dialog {
   backdrop-filter: none !important;
 }
 
+body.is-desktop-runtime .pdf-preview-dialog--flush {
+  padding: 0 !important;
+}
+
+body.is-desktop-runtime .pdf-preview-dialog--flush .el-dialog__body {
+  height: 100%;
+  padding: 0 !important;
+  background: transparent !important;
+}
+
+body.is-desktop-runtime .pdf-preview-dialog--flush .pdf-viewer {
+  box-sizing: border-box;
+  overflow: hidden;
+  border: 1px solid rgba(148, 163, 184, 0.42);
+  border-radius: 12px;
+  box-shadow:
+    0 28px 84px rgba(15, 23, 42, 0.26),
+    0 1px 0 rgba(255, 255, 255, 0.82) inset;
+}
+
 body.is-desktop-runtime .el-message-box {
   width: min(420px, calc(100vw - 72px));
   padding: 0;
@@ -1105,7 +1126,8 @@ body.is-desktop-runtime .el-message-box__headerbtn .el-message-box__close {
   color: #64748b;
 }
 
-body.is-desktop-runtime .el-dialog__body {
+/* 低优先级桌面基线：普通弹窗保留间距，带专属布局的弹窗可直接覆盖。 */
+body.is-desktop-runtime :where(.el-dialog__body) {
   padding: 14px;
   color: #334155;
   font-size: 13px;