fix(components): CSS 변수명 수정 + dead code 제거

- --border-line → --line (5개 컴포넌트 8곳)
- --gradient-accent → --grad-accent (FAB)
- --text-default → --text (MobileSheet)
- useSwipe.js 삭제 (미사용 dead code)

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

src/hooks/useSwipe.js

@@ -1,13 +0,0 @@
-import { useSwipeable } from 'react-swipeable';
-
-export function useSwipe({ onSwipedLeft, onSwipedRight, threshold = 50 } = {}) {
-  const handlers = useSwipeable({
-    onSwipedLeft,
-    onSwipedRight,
-    delta: threshold,
-    trackMouse: false,
-    preventScrollOnSwipe: true,
-  });
-
-  return handlers;
-}