Desktop app (macOS): restore the copy button on the file pane

## What happened

In the Claude desktop app's Code tab on macOS, the file pane, where files attached in the chat
are previewed, had a copy button that put the file's raw contents on the clipboard. That
button is gone. To paste a markdown file into another app, I now have to ask Claude to run
`pbcopy` for me, or open the file in a separate editor and select it there.

## What I want

Put the copy button back on the file pane: one click, raw file contents on the clipboard.

## My position

I am aware this was an intentional change, and I see it as a deliberate removal of my agency.
The copy button let me act on my own files myself. Without it, a one-click task I used to do
on my own now has to go through the AI as a request. Turning a direct user action into a
request to the model takes control away from the user, and I object to it.

## Environment

- Claude desktop app 1.52386.6, Code tab
- Claude Code 2.1.271
- macOS 26.4.1

## Related

- #92759: same missing button, macOS
- #92682: same missing button, Windows
- #81472: copy/paste tracking issue