Add device-code onboarding and ChatGPT token refresh to app-server TUI (#14952)

## Summary
- add device-code ChatGPT sign-in to `tui_app_server` onboarding and
reuse the existing `chatgptAuthTokens` login path
- fall back to browser login when device-code auth is unavailable on the
server
- treat `ChatgptAuthTokens` as an existing signed-in ChatGPT state
during onboarding
- add a local ChatGPT auth loader for handing local tokens to the app
server and serving refresh requests
- handle `account/chatgptAuthTokens/refresh` instead of marking it
unsupported, including workspace/account mismatch checks
- add focused coverage for onboarding success, existing auth handling,
local auth loading, and refresh request behavior

## Testing
- `cargo test -p codex-tui-app-server`
- `just fix -p codex-tui-app-server`

codex-rs/tui_app_server/src/lib.rs

@@ -100,6 +100,7 @@ pub mod insert_history;
 mod key_hint;
 mod line_truncation;
 pub mod live_wrap;
+mod local_chatgpt_auth;
 mod markdown;
 mod markdown_render;
 mod markdown_stream;