Add `copy` button for exported html

[draftman9]

Is your feature request related to a problem? Please describe.
There is a copy button when I use vnote to render .md files. However the button disappears when I export it as .html file.

[tamlok]

This is expected for now because we want to keep the dependency of the exported HTML file as simple as possible. May add it once I found a simple enough implementation of the copy button.

[draftman9]

Maybe this conversation can help you: https://gemini.google.com/share/8e27e9c2015c