Add a Ability to Share/Export a Chat

- [ ] Export Chat button Should collect the chat and download it in text
- [ ] Chat should be in Markdown in the following format
- [ ] Sharing a Chat should be available at a public link

```console
[1:04 PM, 1/20/2025] Bot: heres a response....
[1:29 PM, 1/20/2025] User: How to build something
[1:47 PM, 1/20/2025] Bot: updated readme and env.example
[1:50 PM, 1/20/2025] User: Some prompt from user..
```

Start from the chat UI and identify how the current conversation is represented. Use the checklist and sample format as the acceptance criteria: an Export Chat action downloads the conversation as Markdown/text, and a shared chat can be opened through a public link.

Written by the indexing model from the issue text.