docs: translate blog content(react 19 upgrade guide)

[y0ungchoi]

이슈번호: #1307

react 19 upgrade guide 번역

• 블로그 포스트 React 19 Upgrade Guide를 번역
• 다른 포스트에 사용된 "리듀서"를 "reducer"로 통일 -> API 번역 문서와 다른 포스트에서도 reducer로 주로 사용되고 있습니다.

필수 확인 사항

• 기여자 행동 강령 규약^{Code of Conduct}
• 기여 가이드라인^Contributing
• 공통 스타일 가이드^{Universal Style Guide}
• 번역을 위한 모범 사례^{Best Practices for Translation}
• 번역 용어 정리^{Translate Glossary}
• textlint 가이드^{Textlint Guide}
• 맞춤법 검사^{Spelling Check}

선택 확인 사항

• 번역 초안 작성^{Draft Translation}
• 리뷰 반영^{Resolve Reviews}

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[github-actions]

Size changes

📦 Next.js Bundle Analysis for undefined

This analysis was generated by the Next.js Bundle Analysis action. 🤖

Five Pages Changed Size

The following pages changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/404	130.53 KB (-5 B)	240.98 KB
/500	130.53 KB (-5 B)	240.98 KB
/[[...markdownPath]]	132.6 KB (-5 B)	243.04 KB
/errors	130.67 KB (-5 B)	241.12 KB
/errors/[errorCode]	130.65 KB (-5 B)	241.09 KB

Details

Only the gzipped size is provided here based on an expert tip.

First Load is the size of the global bundle plus the bundle for the individual page. If a user were to show up to your website and land on a given page, the first load size represents the amount of javascript that user would need to download. If next/link is used, subsequent page loads would only need to download that page's bundle (the number in the "Size" column), since the global bundle has already been downloaded.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

Next to the size is how much the size has increased or decreased compared with the base branch of this PR. If this percentage has increased by 10% or more, there will be a red status indicator applied, indicating that special attention should be given to this.

[lumirlumir]

LGTM. 기여 감사합니다.

[y0ungchoi]

@lumirlumir 꼼꼼하게 봐주셔서 감사합니다! 다음에는 수정해 주신 부분들을 유의해서 번역하도록 하겠습니다 :)