Migrate stacks/account.go from survey to huh

[blsmth]

Summary

Migrate the account administrators prompt in stacks/account.go from survey to huh.

Details

• Complexity: Low
• Prompts: 1 Multiline (admin emails)
• Single survey.Multiline via ui.AskQuestions with MultiLineValueProxy
• No AWS data dependencies

Checklist

• Extract form builder function (pure, no AWS mocks needed)
• Replace ui.AskQuestions call with huh form
• Add teatest + ui/uitest tests
• Add VHS recording for visual verification
• Verify non-interactive mode still works