Add client-side validation for notification channel forms

[Scarage1]

Currently users can create channels without required fields. Add validation based on channel type.\n\nFiles:\n- frontend/src/components/NotificationChannelConfig.tsx\n\nRules:\n- Email: recipients required\n- Webhook: url required\n- Slack: webhook_url required\n\nTasks:\n- Inline error text\n- Disable Create/Update when invalid\n\nAcceptance Criteria:\n- Invalid form cannot be submitted\n- Clear error message displayed