feat(web): add conversation export as Markdown and JSON (#1002)

* feat(web): add conversation export as Markdown and JSON (#976)

Add the ability to export conversations in Markdown and JSON formats,
accessible from both the chat header and the sidebar context menu.

- Add export utility (formatThreadAsMarkdown, formatThreadAsJSON) with
  support for user/assistant messages, thinking blocks, and tool calls
- Add ExportTrigger component in chat header (appears when messages exist)
- Add Export submenu to sidebar dropdown (fetches full thread state on demand)
- Add i18n translations for en-US and zh-CN

Closes #976

Made-with: Cursor

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update thread creation timestamp to updated_at

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

frontend/src/components/workspace/export-trigger.tsx

@@ -0,0 +1,81 @@
+"use client";
+
+import { Download, FileJson, FileText } from "lucide-react";
+import { useCallback } from "react";
+import { toast } from "sonner";
+
+import { Button } from "@/components/ui/button";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from "@/components/ui/dropdown-menu";
+import { useI18n } from "@/core/i18n/hooks";
+import {
+  exportThreadAsJSON,
+  exportThreadAsMarkdown,
+} from "@/core/threads/export";
+import type { AgentThread } from "@/core/threads/types";
+
+import { useThread } from "./messages/context";
+import { Tooltip } from "./tooltip";
+
+export function ExportTrigger({ threadId }: { threadId: string }) {
+  const { t } = useI18n();
+  const { thread } = useThread();
+
+  const messages = thread.messages;
+
+  const handleExport = useCallback(
+    (format: "markdown" | "json") => {
+      if (messages.length === 0) {
+        toast.error(t.conversation.noMessages);
+        return;
+      }
+      const agentThread = {
+        thread_id: threadId,
+        updated_at: new Date().toISOString(),
+        values: thread.values,
+      } as AgentThread;
+
+      if (format === "markdown") {
+        exportThreadAsMarkdown(agentThread, messages);
+      } else {
+        exportThreadAsJSON(agentThread, messages);
+      }
+      toast.success(t.common.exportSuccess);
+    },
+    [messages, thread.values, threadId, t],
+  );
+
+  if (messages.length === 0) {
+    return null;
+  }
+
+  return (
+    <DropdownMenu>
+      <Tooltip content={t.common.export}>
+        <DropdownMenuTrigger asChild>
+          <Button
+            className="text-muted-foreground hover:text-foreground"
+            variant="ghost"
+          >
+            <Download />
+            {t.common.export}
+          </Button>
+        </DropdownMenuTrigger>
+      </Tooltip>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onSelect={() => handleExport("markdown")}>
+          <FileText className="text-muted-foreground" />
+          <span>{t.common.exportAsMarkdown}</span>
+        </DropdownMenuItem>
+        <DropdownMenuItem onSelect={() => handleExport("json")}>
+          <FileJson className="text-muted-foreground" />
+          <span>{t.common.exportAsJSON}</span>
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}