Fix Markdown copy script

[akaihola]

Fixes #6

• [x] fix TypeError: e is null
• [ ] fix empty content of _Prompt:_ and _Claude:_

[akaihola]

Changing pt-16 to pt-1 in the chat content selector got rid of the e is null error message. But the output is still empty:

# <title of the Claude chat>
`2024-07-07 11:49:13`

_Prompt_:
_Claude_:

[akaihola]

The hierarchy of elements inside div.font-user-message and div.font-claude-message seems to have changed and broken the script.