Adds a --device-code flag to `roo auth login` that implements a device
code authentication flow similar to GitHub CLI. This allows users on
remote or headless servers to authenticate by:
1. Running `roo auth login --device-code`
2. Opening a verification URL on any device with a browser
3. Entering the displayed user code
4. CLI polls for completion and saves the token
The server-side endpoints (POST /api/cli/device-code and
POST /api/cli/device-code/poll) need to be implemented separately.
Closes#11925
