Add copy shortcut to onboarding auth link and device code screens

Summary

The onboarding auth flow exposes browser-link and device-code screens, but there is no keyboard shortcut to copy the auth link or device code from those screens.

This is awkward in remote/headless setups, where users often need to move the URL or device code to another device manually.

Current behavior

In the onboarding auth flow:

• Continue in browser shows the auth URL, but there is no copy action.
• Device code shows the verification URL and user code, but there is no copy action.

Proposed behavior

Add a c shortcut on both screens:

• On the browser-link screen, c copies the auth URL.
• On the device-code screen, c copies the device user code.
• Show an inline hint such as Press c to copy ....
• Show transient success/failure feedback after the copy attempt.

This should be reflected in both:

• codex-rs/tui
• codex-rs/tui_app_server

Why this seems worth adding

• Improves usability in remote/headless environments.
• Reduces error-prone manual transcription of URLs/codes.
• Fits the existing keyboard-driven TUI interaction model.
• The repo already has clipboard support that can be reused.

Implementation notes

I validated a local implementation that:

• Reuses the existing clipboard text helper.
• Adds the c key handling only when one of these auth screens is active.
• Keeps the change local to onboarding auth rendering/state.
• Adds snapshot coverage for the browser-link screen.

I also mirrored the behavior in both ui and ui_app_server.

Validation performed locally

• cargo test -p codex-tui
• cargo test -p codex-tui-app-server
• just fix -p codex-tui
• just fix -p codex-tui-app-server
• scoped argument-comment lint run for the touched crates
• just fmt

Reference implementation

I have a working branch on my fork in case maintainers want to review or request a PR:

• Branch: copy-auth-prompt
• Compare view: https://github.com/tzarebczan/codex/compare/main...copy-auth-prompt