Add refresh button to PR drawer

[Umaaz]

Summary

• Adds a refresh button (↻) to the PR drawer header that force-refreshes PR details, comments, and build status
• Refactors fetch logic from a useEffect with refreshKey counter into a useCallback for cleaner reuse between initial load and manual refresh

Test plan

• Open a PR drawer and verify details load as before
• Click the refresh button and confirm the loading state appears and data reloads
• Verify the button appears in the header between the PR number and the GitHub external link

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