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/app/workspace/chats/[thread_id]/page.tsx

@@ -9,6 +9,7 @@ import {
   useSpecificChatMode,
   useThreadChat,
 } from "@/components/workspace/chats";
+import { ExportTrigger } from "@/components/workspace/export-trigger";
 import { InputBox } from "@/components/workspace/input-box";
 import { MessageList } from "@/components/workspace/messages";
 import { ThreadContext } from "@/components/workspace/messages/context";
@@ -84,7 +85,8 @@ export default function ChatPage() {
             <div className="flex w-full items-center text-sm font-medium">
               <ThreadTitle threadId={threadId} thread={thread} />
             </div>
-            <div>
+            <div className="flex items-center">
+              <ExportTrigger threadId={threadId} />
               <ArtifactTrigger />
             </div>
           </header>