[task] Render GitHub workflow queue in TUI

[phongndo]

Goal

Replace the placeholder TUI dashboard with the first GitHub workflow queue view.

Why this matters

The TUI is one of Nexgit's main product surfaces. It should prove that app-server/workflow data can become a useful keyboard-first local GitHub dashboard.

Command/API target

Use shared Rust workflow data for issues and PRs once the v0.1 app-server data methods exist.

Depends on

• [task] Add app-server auth and repository status methods #7 for auth/repo state.
• [task] Add app-server issue list and view methods #8 for issue data.
• [task] Add app-server PR list and view methods #9 for PR data.

Out of scope

• Do not implement checks or review-detail rendering in this issue.
• Do not call GitHub directly from UI rendering code.
• Do not build desktop UI here.

Definition of done

• TUI shows current repo/auth state.
• TUI shows a combined or tabbed issue/PR queue from shared workflow data.
• Loading, no-auth, no-repo, empty, and error 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 in the PR description.

Contributor notes

Keep the layout simple. This issue is the first useful TUI surface, not the final product design.