Implement in-app upgrade flow for new releases

[germanescobar]

Summary

Add an app upgrade flow that monitors for newly published releases and gives users a convenient way to update the Electron app.

Goals

• Detect when a newer app release is available.
• Notify the user without interrupting active work.
• Provide a clear, low-friction upgrade action from inside the app.
• Preserve useful error context when update checks or downloads fail.

Suggested approach

• Use the app version from package.json / Electron runtime as the installed version.
• Check GitHub Releases for germanescobar/coding-orchestrator on startup and periodically while the app is running.
• Compare the latest release version with the installed version.
• Surface update availability in the UI, likely in settings and/or a subtle app-level notification.
• For macOS, decide whether the first implementation should:
  • open the release download page in the browser, or
  • support a fuller auto-update flow using an Electron updater package.

Acceptance criteria

• The app can determine the currently installed version.
• The app can check for the latest available release.
• If a newer release exists, the user is offered a visible upgrade action.
• If the app is already current, update checks report that cleanly.
• Network/API failures are handled deliberately and do not crash the app.
• The behavior is covered by focused tests for version comparison and update-check result handling.

Notes

For the initial Mac-only build, a simple "new version available" prompt that opens the release page may be enough. A later iteration can add automatic download/install if we want a native updater experience.