Copy file contents button in the Files panel (desktop/web)

[fionn77]

Idea

Add a Copy contents button (and a matching context-menu item) to the desktop/web Files panel header that copies the whole file's loaded text to the clipboard.

Why

A common flow: the agent prepares a complete config file (for example a full Tailscale policy) and links it in chat. You then need to paste all of it somewhere else, such as an admin console. Selecting it by hand doesn't work well today:

• Drag-selecting a long file in the read-only preview loses the lines that scroll out of the render window ([Bug]: Copying text from the Files panel returns only part of the file #15927).
• On Windows, Ctrl+A in the preview selects the whole app UI, not the file.

Even once #15927 is fixed, a one-click copy is faster and less error-prone than select-all on a long file.

Suggested behaviour

• Button in the Files panel header, next to existing actions such as Copy path. Tooltip: "Copy file contents".
• Copies the raw source text (not rendered Markdown/HTML) from the already-loaded string, so it doesn't depend on which lines are mounted.
• If the preview was truncated at the size limit, disable the button or say so, rather than silently copying a partial file.
• Brief "Copied" confirmation.

Related: #17984 (the same "Copy raw text" action for linked files on iOS), #7481 (Copy Path in the Files panel context menu), #15927.