🎨 Palette: 빈 디렉토리 UI 일관성 및 시각적 개선

[seonghobae]

• .empty-dir 클래스에 flexbox 스타일을 적용하여 주변 컴포넌트와의 간격 및 정렬 일관성 향상
• 빈 디렉토리 메시지에 폴더 아이콘(📂)을 추가하고 접근성을 고려하여 aria-hidden 처리
• testGoEmptyDir에 아이콘 존재 여부를 검증하는 단언문 추가
• 관련된 UX 인사이트를 .jules/palette.md에 한국어로 기록

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PR created automatically by Jules for task 10535098209047678319 started by @seonghobae

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