Signing Key dialog: private key textarea overflows the dialog

## Problem

After creating a new Signing Key (Settings → Embedded → Signing Keys → **New Signing Key**), the **Save your private Signing Key** dialog renders its content outside the dialog panel: the private key textarea, the close button, and the **Copy** / **Done** buttons spill past the right edge of the modal.

## Cause

`client/src/ee/pages/settings/embedded/signing-keys/components/SigningKeyDialog.tsx` renders the key in the shared `Textarea` with `text-nowrap`. Since the shadcn/Tailwind v4 update, `Textarea` uses `field-sizing-content`, so the textarea grows to the width of its longest PEM line instead of honouring `rows`. `DialogContent` is a CSS grid whose items default to `min-width: auto`, so the wide textarea widens the whole form beyond the dialog.

Additionally, the **Copy** button is inside the `` without `type="button"`, so clicking it also submits the form.

## Expected

The private key stays inside the dialog in a fixed-size, scrollable textarea, and **Copy** only copies the key.