Show and update the installed Codex CLI

[squarepots]

Summary

• Show the version of the Codex CLI that GSwitch actually launches, with an explicit update action and an official install guide.
• Delegate updates to the CLI's supported codex update command. On Windows npm installs, preserve the official launcher's install context when GSwitch starts the native executable directly.
• Keep CLI checks and update errors in a localized dialog; block updating while Codex or another GSwitch operation is active. Saved accounts and credentials are not changed.

Verification

• pnpm exec vitest run src/App.test.tsx (64 passed)
• pnpm exec vitest run src/i18n.test.ts (3 passed)
• pnpm build
• Focused Rust tests for a fake CLI and the Windows npm native launcher; cargo fmt --check
• No update of the maintainer's installed CLI and no real-account operation

Related to #126. The issue remains open until the feature is in a public Windows release.