Add a 'Copy Screenshot' / download button to ScreenPanel

[alimaandev]

Summary

desktop/src/components/sidebar/ScreenPanel.tsx renders a screenshot from the backend but there is no way to save the image to disk from the UI.

Goal

Add a small 'Download' button in the ScreenPanel header that triggers a download of the current screenshot image.

Tasks

• Add a download button next to the refresh button
• Implement download via an anchor with \download\ attribute + blob URL (or fetch the image URL directly)
• Include the timestamp in the filename (e.g. \screenshot-2026-08-20.png)
• Add/extend a Vitest test in desktop/src/test/ScreenPanel.test.tsx

Acceptance criteria

• Clicking Download saves the current screenshot with a timestamped filename
• Button disabled when no screenshot is loaded
• Test covers the click → download trigger

Labels

good first issue, help wanted