Add richer PR details to the changes pane

[penso]

Summary

• add a shared PR summary card above the changes list and reuse its status presentation in the sidebar hover UI
• show richer GitHub PR state, including checks, review state, mergeability, loading feedback, and expandable check details
• fetch PR details for worktrees in parallel and precompute check ordering/counts so the UI render path avoids blocking I/O and per-render sorting

Testing

• just format
• just lint
• cargo +nightly-2025-11-30 test -p arbor-gui prioritized_pr_checks_show_failures_before_pending_before_successes
• cargo +nightly-2025-11-30 test -p arbor-gui in_progress_checks_remain_pending_without_conclusion

[codspeed-hq]

Merging this PR will not alter performance

✅ 21 untouched benchmarks

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_{Comparing side-pane-pr-details (218d064) with main (7204e25)}