tui: route device-code auth through app server (#16827)

Addresses #7646
Also enables device code auth for remote TUI sessions

Problem: TUI onboarding handled device-code login directly rather than
using the recently-added app server support for device auth. Also, auth
screens kept animating while users needed to copy login details.

Solution: Route device-code onboarding through app-server login APIs and
make the auth screens static while those copy-oriented flows are
visible.

codex-rs/tui/src/local_chatgpt_auth.rs

@@ -1,3 +1,5 @@
+#![cfg(test)]
+
 use std::path::Path;
 
 use codex_app_server_protocol::AuthMode;