fix(deepfield): home 그리드 지그재그 3-wide 배치(빈 칸 제거) + 데드 CSS 정리

- ShowcaseGrid: index 0·3·4 → feature/col-span-2, 1·2·5 → standard
  wide 3장+standard 3장 = 9셀(3×3) 완전 충전, Row 2 col3 빈 칸 제거
- ShowcaseCard: ring-1(인라인 boxShadow에 덮이는 데드 클래스) 제거
  transition-[...]에서 미사용 border-color 제거

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/components/deepfield/ShowcaseCard.tsx

@@ -249,7 +249,7 @@ export default function ShowcaseCard({ slot, size = 'standard', index }: Props)
       onMouseLeave={() => setHovered(false)}
       className={[
         'group/card relative isolate h-full w-full overflow-hidden rounded-2xl',
-        'ring-1 transition-[transform,box-shadow,border-color] duration-500',
+        'transition-[transform,box-shadow] duration-500',
         '[transition-timing-function:cubic-bezier(0.16,1,0.3,1)]',
         'motion-safe:hover:scale-[1.03]',
         isLink ? 'cursor-pointer' : 'cursor-default',