Validate registry URL and OAuth config eagerly on set/login

[eleftherias]

Summary

Registry URL and OAuth configuration errors currently surface only when the user first lists servers — well after thv registry set or thv registry login have returned successfully. The configuration appears saved correctly, but the error appears later through an unrelated-looking message.

This issue tracks moving those validations earlier so users get immediate, actionable feedback at configuration time.

What needs to change

• Probe the registry API URL before saving it. Verify the endpoint is reachable before persisting the URL. A 401 response is acceptable only when auth credentials are being provided in the same operation; any other failure should be returned immediately.

Generated with Claude Code

[eleftherias]

Closing this issue because it needs more investigation to validate where the problem is. Will reopen if needed.