Add a loading state to ScreenPanel screenshot refresh

[alimaandev]

Summary

desktop/src/components/sidebar/ScreenPanel.tsx shows the screen screenshot from the API. When the user triggers a refresh/recapture, there is no visual loading state between clicks, so the UI feels unresponsive.

Goal

Show a Skeleton placeholder (reuse desktop/src/components/common/Skeleton.tsx) over the screenshot area while a new capture is being fetched, and disable the refresh button during the fetch.

Tasks

• Track a
  efreshing\ boolean driven by the fetch lifecycle (prop or local state)
• Render skeleton while refreshing; show image when ready
• Disable + dim the refresh button during the fetch
• Add/extend a Vitest test in desktop/src/test/ScreenPanel.test.tsx

Acceptance criteria

• Refresh button disabled while fetching
• Skeleton shown instead of stale image during refresh
• Test covers the refreshing state

Labels

good first issue, help wanted