## 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`
This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.
Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.
Eric Traut