🎨 Palette: 비동기 작업 버튼에 명시적인 로딩 상태 추가

[seonghobae]

💡 What: demo.js의 비동기 작업 버튼(새로고침, 문서 제출, 재시도 등)이 네트워크 요청 중일 때 로딩 상태 텍스트("Submitting...", "Refreshing...", "Retrying...")를 표시하고 비활성화되도록 수정했습니다.
🎯 Why: 비동기 작업 시 명시적인 시각적 피드백이 없어 사용자가 혼란을 느끼거나 중복 제출(double-submission)하는 문제를 방지하기 위함입니다.
♿ Accessibility: 스크린 리더 및 키보드 사용자가 버튼 상태 변화(비활성화)를 명확하게 인지할 수 있도록 접근성을 개선했습니다.

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PR created automatically by Jules for task 8580953324463905925 started by @seonghobae

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