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/core/i18n/locales/en-US.ts

@@ -43,6 +43,10 @@ export const enUS: Translations = {
     save: "Save",
     install: "Install",
     create: "Create",
+    export: "Export",
+    exportAsMarkdown: "Export as Markdown",
+    exportAsJSON: "Export as JSON",
+    exportSuccess: "Conversation exported",
   },
 
   // Welcome