[task] Render TUI PR detail and checks view

[phongndo]

Goal

Show selected PR details and checks in the TUI.

Why this matters

A PR queue is only useful if maintainers can quickly see whether a PR is blocked by CI, pending review, or ready for follow-up.

Command/API target

Render github.pr.view and github.pr.checks data for the selected PR.

Depends on

• [task] Add app-server PR list and view methods #9 for PR detail data.
• [task] Add app-server PR checks method #11 for PR checks data.
• TUI PR list issue for selection behavior.

Out of scope

• Do not implement check reruns.
• Do not implement review-thread detail rendering.
• Do not build desktop UI.

Definition of done

• TUI can show PR title, author, state, branch/base, and URL.
• TUI can show check names and states for the selected PR.
• Missing or unavailable check data is understandable.

Verification

• cargo fmt --all --check
• cargo check --workspace
• Run cargo run -p nexgit-cli -- tui and include a screenshot or concise manual observation.

[github-actions]

This issue needs a small update before triage:

• Area must be one of Automation, CLI, Core, Desktop, Design, Docs, Protocol, Tests, or TUI so triage automation can route the issue.

Please edit the issue instead of opening a new one.

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Potentially related existing work:

• [task] Render GitHub workflow queue in TUI #13 [task] Render GitHub workflow queue in TUI (open)
• [task] Render GitHub PR list in TUI #17 [task] Render GitHub PR list in TUI (open)
• [task] Build desktop PR list and detail panel #21 [task] Build desktop PR list and detail panel (open)

If one of these is the canonical thread, comment there and a maintainer can close this issue with /duplicate #123; otherwise a maintainer can clear the review with /not-duplicate.