Add Copy button to `<Output>` control

[ai]

We use <Output> control on Signup page to allow user to copy random user ID / secret. We will have better UX if we will have a copy button directly.

1. Start visual tests and open Output story.
2. Edit web/ui/output.svelte
3. Add size="icon" button with copy icon.
4. After click change icon to OK icon for a few seconds (use aria-live to announce copied for screen readers).
5. Copy text by using async Clipboard API.
6. Add OK state to Output story.
7. Open Signup page story and check that everything works.