[task] Render TUI review comments view

[phongndo]

Goal

Show PR review comments or review threads in the TUI.

Why this matters

Review context is one of the workflow areas that often sends users back to GitHub web. Nexgit should make unresolved review work visible locally.

Command/API target

Render github.pr.reviewThreads data for the selected PR.

Depends on

• [task] Add app-server review comments and threads method #12 for review comments/thread data.
• TUI PR detail issue for selected PR context.

Out of scope

• Do not implement replying, resolving, or submitting reviews.
• Do not build a full diff viewer.
• Do not build desktop UI.

Definition of done

• TUI can show review thread/comment summaries for a selected PR.
• File path, line/position, author, body excerpt, and resolved state appear where available.
• Empty, loading, permission, and unsupported-data states are readable.

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.