Username availability check missing in Setup Wizard admin step

Description:

In AdminInfoStep.tsx, the username validation was missing an API check to verify if the username is already taken:

Expected Behavior

validateUsernameshould call the users.checkUsernameAvailabilityAPI endpoint and return an error message if the username is already in use.

Current Behavior

// TODO: check if username exists
const validateUsername = (username: string): boolean | string => {
	if (!usernameRegExp.test(username) || hasBlockedName(username)) {
		return t('Invalid_username');
	}

	return true;
};

Steps to Reproduce

See packages/ui-client/src/views/setupWizard/steps/AdminInfoStep.tsx

Additional Context

• checkUsernameAvailability endpoint GET /v1/users.checkUsernameAvailability is already available in the API
• No runtime behavior changes beyond the added API call
• The following should be changed in AdminInfoStep.tsx, SetupWizardContext.ts, SetupWizardProvider.tsx

Start with packages/ui-client/src/views/setupWizard/steps/AdminInfoStep.tsx and trace the username validation flow into SetupWizardContext.ts and SetupWizardProvider.tsx. Confirm how the existing users.checkUsernameAvailability endpoint is called, then verify that an already-used username produces an error while valid available names still pass validation.