[task] Add desktop app-server loading and error states

[phongndo]

Goal

Make desktop app-server startup, loading, and error states clear and actionable.

Why this matters

The desktop app depends on the Rust app-server. Users need to know whether Nexgit is loading, missing auth, outside a repo, blocked by app-server startup, or failing due to GitHub/API errors.

Command/API target

Improve renderer state handling around typed preload/app-server requests.

Depends on

• [task] Add app-server auth and repository status methods #7 for auth/repo states.
• [task] Render GitHub workflow dashboard in desktop #14 for desktop dashboard structure.

Out of scope

• Do not add new GitHub data methods.
• Do not run GitHub calls from renderer code.
• Do not redesign the full app shell.

Definition of done

• Desktop distinguishes app-server starting, app-server failed, GitHub no-auth, no-repo, empty data, and normal loaded states.
• Errors are readable and do not collapse the whole screen into raw stack traces.
• Retry or refresh behavior is either implemented or clearly deferred.

Verification

• pnpm desktop:typecheck
• pnpm desktop:build
• Run pnpm desktop:dev with normal and forced app-server failure conditions; include screenshots or observations.

[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.

---

Potentially related existing work:

• [task] Build desktop issue list panel #20 [task] Build desktop issue list panel (open)
• [task] Render GitHub workflow dashboard in desktop #14 [task] Render GitHub workflow dashboard in desktop (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.