* removes useless "no items" pop-up and does not show any menu at all if
no actions are applicable.
This required adding `pub fn maybe_menu` into the 

* adds actions to copy rendered text and markdown text: in fact,
`cmd/ctrl-c` work already but we did not expose it

Before:


https://github.com/user-attachments/assets/906f2bfd-bf19-48de-bbb5-85bb33431f60

After:


https://github.com/user-attachments/assets/2c039c4d-2d42-4689-9ac1-12e3b10119cf

Release Notes:

- Improved context menu experience in the markdown preview