🎨 Palette: 빈 디렉토리 상태 메시지 접근성 개선 (role="status")

[seonghobae]

💡 What: src/main/kotlin/html4tree/main.kt의 빈 디렉토리 메시지(<div class="empty-dir">)에 role="status" 속성을 추가했습니다.
🎯 Why: 자동으로 생성된 디렉토리 트리가 비어 있을 때 스크린 리더 사용자가 현재 위치의 상태(빈 목록)를 포커스 이동 없이 명확히 안내받을 수 있도록 하기 위함입니다.
📸 Before/After: 시각적 변경 없음 (HTML 요소 속성만 추가)
♿ Accessibility: 상태 메시지 요소를 제공하여 시각 장애가 있는 사용자가 화면 내용을 파악하기 쉽게 접근성이 개선되었습니다.

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PR created automatically by Jules for task 6415401606601411389 started by @seonghobae

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