feat(Card): add ghost variant with transparent background and no border

[osohyun0224]

Fixed #264

Changes

Card 컴포넌트에 ghost variant를 추가했습니다. 실제 SIPE 공식 홈페이지 코드로도 테스트 완료했습니다!

• Card.css.ts: CardVariant에 ghost 옵션 추가 (배경색: transparent, border: none, padding: 0)
• Card.test.tsx: ghost variant에 대한 테스트 3개 추가 (배경색, border, padding 검증)
• Card.stories.tsx: argTypes에 ghost 옵션 추가 및 GhostVariant 스토리 추가 (모든 ratio 조합 포함)
• .changeset/add-card-ghost-variant.md: @sipe-team/card minor 버전 changeset 추가

Visuals

Checklist

• Have you written the functional specifications?
• Have you written the test code?

Additional Discussion Points

• ghost variant는 배경과 테두리가 없는 투명한 카드로, 레이아웃 용도나 콘텐츠만 감싸는 경우에 활용할 수 있습니다.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 18cd9aa

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@sipe-team/card	Minor
@sipe-team/side	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
packages/card/src/Card.css.ts	100.00% <ø> (ø)
packages/card/src/Card.test.tsx	100.00% <100.00%> (ø)

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