auth login needs explicit plain/no-browser mode

## Describe the Bug

`pangolin-cli auth login` relies on terminal/browser auto-detection. In containerized rootful Podman usage that is not deterministic enough: the login flow can start without visibly printing the one-time code, leaving the user stuck in a broken TUI path.

A plain-text-capable path exists, but there is no explicit CLI flag to force it.

## Environment

- OS Type & Version: Linux host using rootful Podman from an interactive shell
- Pangolin Version: n/a
- Gerbil Version: n/a
- Traefik Version: n/a
- Newt Version: n/a
- Client Version: 0.8.0

## To Reproduce

1. Run `podman run --rm -it -e PANGOLIN_ENDPOINT=https://vpn.somendpoint.io ghcr.io/fosrl/cli auth login`
2. In some real-shell/container contexts, login starts but the device code is not rendered visibly.
3. Compare with `printf "
" | podman run --rm -i -e PANGOLIN_ENDPOINT=https://vpn.somendpoint.io -e TERM=dumb -e BROWSER=true ghcr.io/fosrl/cli auth login`
4. That path prints plain-text device-login output.

## Expected Behavior

Please add an explicit non-TUI login mode instead of relying on auto-detection.

Suggested flags:
- `--plain`
- `--no-browser`
- ideally `--json`

Stable output such as `DEVICE_CODE=...` and `DEVICE_LOGIN_URL=...`, or JSON, would make container use reliable.