feat(table): 表格下载支持 CSV 和 Markdown 双格式下拉选择

重构下载功能，将单一下载按钮改为 DropdownMenu 下拉菜单，新增 CSV 导出和 escapeCsvCell 辅助函数，downloadTextFile 支持自定义 MIME 类型。

frontend/src/components/workspace/messages/markdown-content.tsx

@@ -12,6 +12,12 @@ import {
   MessageResponse,
   type MessageResponseProps,
 } from "@/components/ai-elements/message";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from "@/components/ui/dropdown-menu";
 import { useI18n } from "@/core/i18n/hooks";
 import { streamdownPlugins } from "@/core/streamdown";
 import { CopyButton } from "@/components/workspace/copy-button";
@@ -58,9 +64,22 @@ function toMarkdownTable(data: TableData): string {
   return [headerLine, dividerLine, ...rowLines].join("\n");
 }
 
-function downloadMarkdownFile(content: string, filename: string) {
+function escapeCsvCell(cell: string): string {
+  const normalized = cell.replace(/\r\n/g, "\n");
+  if (!/["\n,]/.test(normalized)) return normalized;
+  return `"${normalized.replace(/"/g, '""')}"`;
+}
+
+function toCsvTable(data: TableData): string {
+  if (data.headers.length === 0) return "";
+  const headerLine = data.headers.map(escapeCsvCell).join(",");
+  const rowLines = data.rows.map((row) => row.map(escapeCsvCell).join(","));
+  return [headerLine, ...rowLines].join("\r\n");
+}
+
+function downloadTextFile(content: string, filename: string, mimeType: string) {
   const blob = new Blob(["\uFEFF", content], {
-    type: "text/markdown;charset=utf-8",
+    type: mimeType,
   });
   const url = URL.createObjectURL(blob);
   const anchor = document.createElement("a");
@@ -99,13 +118,23 @@ export function MarkdownTable({
   })();
 
   const handleDownload = useCallback(() => {
+    const table = tableRef.current;
+    if (!table) return;
+    const data = parseTableData(table);
+    if (!data) return;
+    const csv = toCsvTable(data);
+    if (!csv) return;
+    downloadTextFile(csv, "table.csv", "text/csv;charset=utf-8");
+  }, []);
+
+  const handleDownloadMarkdown = useCallback(() => {
     const table = tableRef.current;
     if (!table) return;
     const data = parseTableData(table);
     if (!data) return;
     const markdown = toMarkdownTable(data);
     if (!markdown) return;
-    downloadMarkdownFile(markdown, "table.md");
+    downloadTextFile(markdown, "table.md", "text/markdown;charset=utf-8");
   }, []);
 
   return (
@@ -115,15 +144,24 @@ export function MarkdownTable({
     >
       <div className="flex items-center justify-end gap-1">
         <CopyButton className="text-muted-foreground hover:bg-transparent hover:text-foreground cursor-pointer p-1 transition-all" clipboardData={clipboardData} />
+        <DropdownMenu>
           <Tooltip content={downloadLabel}>
+            <DropdownMenuTrigger asChild>
               <button
                 className="h-[32px] w-[32px] text-muted-foreground hover:text-foreground cursor-pointer p-1 transition-all"
-            onClick={handleDownload}
                 type="button"
               >
                 <DownloadIcon size={16} />
               </button>
+            </DropdownMenuTrigger>
           </Tooltip>
+          <DropdownMenuContent align="end" className="min-w-[140px] p-1">
+            <DropdownMenuItem onSelect={handleDownload}>CSV</DropdownMenuItem>
+            <DropdownMenuItem onSelect={handleDownloadMarkdown}>
+              Markdown
+            </DropdownMenuItem>
+          </DropdownMenuContent>
+        </DropdownMenu>
       </div>
       <div className="overflow-x-auto">
         <table