Desktop app: file pane lost its copy button; ⌘A selects the chat too

## Summary

In the Claude desktop app (Code tab), the file viewer pane on the right side of the screen no longer offers a way to copy its contents on its own. Until recently there was a copy control at the top right of that pane, and clicking the file's header also let me copy the file. Both are gone.

## What happens now

- The pane's top-right row shows search, a three-dot menu, open-elsewhere, expand, and close. None of them copies the file.
- Right-clicking inside the pane shows a small menu with "Copy ⌘C" and "Select All ⌘A", but Select All (⌘A) selects the chat transcript on the left as well as the file on the right, so the clipboard ends up with both.
- There is no way to copy just the file shown in the right pane without dragging a selection by hand.

## What I expect

A copy button on the file pane, like the one in the Codex desktop app: one click copies the whole file shown in that pane and nothing else. Clicking the file header to copy, which used to work, would also be fine.

## Why it matters

I move long prompts and handover documents between this app and other tools every day. Losing a one-click copy on the file pane turns a routine action into a fiddly drag-select. This is one of the reasons I am weighing moving my work to the Codex app, whose file pane has this button.

## Environment

- Claude desktop app 1.46388.4, Code tab
- macOS 26.5.2
- Model in session: Fable 5.1

Start in the Claude desktop app's Code tab and reproduce the file-pane behavior on macOS 26.5.2 with app version 1.46388.4, checking whether ⌘A selects both panes. Done means the file pane has a one-click control that copies only the displayed file, or its header-copy behavior is restored, without chat text on the clipboard.

索引モデルが issue の本文から書いたものです。