Add export chat as Markdown feature

[erkin98]

Summary

• Adds a download button to the chat section toolbar that exports the conversation as a .md file
• Adds an exportChat command accessible from the command palette
• Introduces downloadChatAsMarkdown and formatChatAsMarkdown utility functions in @jupyter/chat

The exported Markdown includes sender names, timestamps, and preserves message formatting. The file downloads directly to the user's machine via a temporary anchor element.

Changes

Package	File	Change
@jupyter/chat	src/utils.ts	New formatChatAsMarkdown and downloadChatAsMarkdown functions
@jupyter/chat	src/index.ts	Re-export downloadChatAsMarkdown
@jupyter/chat	src/widgets/multichat-panel.tsx	Export button in ChatSection toolbar
jupyterlab-chat	src/token.ts	exportChat command ID
jupyterlab-chat-extension	src/index.ts	Command registration + palette entry

Test plan

• Open a chat with several messages, click the download icon — verify .md file downloads
• Open command palette, run "Export chat as Markdown" — same behavior
• Verify exported Markdown has correct sender names and timestamps
• Verify the command is disabled when no chat widget is focused

Closes #145

[github-actions]

👈 Launch a Binder on branch erkin98/jupyter-chat/feat%2Fexport-chat-button

[brichet]

Thanks @erkin98 for working on this, it looks great.

Looks like it picked up some conflicts with main.

Also, the related issue should probably be #145 instead.

[erkin98]

Rebased on main, conflicts are resolved. Fixed the linked issue to #145 too, good catch.