bug(adk-ui): auto-login gets stuck on "Redirecting to login..." when auth server is unreachable

[tomkis]

Describe the bug
When local dev auto-login is enabled and the auth server (Keycloak) is not running, the UI gets stuck on "Redirecting to login..." indefinitely with no error message or recovery option.

To Reproduce

1. Enable local dev auto-login (localtest.me setup)
2. Stop / don't start the Keycloak server
3. Navigate to the app

Expected behavior
User sees an error message indicating the auth server is unreachable, with an option to retry.

Actual behavior
UI is stuck on "Redirecting to login..." — the failed signIn() call is silently swallowed (void signIn() in AutoSignIn) and no feedback is shown.