feat(checkbox): チェックボックスパターンと例の日本語翻訳

[ef81sp]

関連Issue

• close Checkbox #89

レビュワーに見てもらいたい箇所

NA

翻訳チェックリスト

• html要素にlang="ja"属性を付与する
• h1要素の直下に翻訳元リビジョンを記載する

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[Copilot]

Pull Request Overview

This PR translates the checkbox pattern documentation and examples from English to Japanese. The translation follows proper localization practices by updating the HTML lang attribute, adding revision reference comments, and translating all user-facing text content.

• Japanese translation of checkbox pattern documentation and two example files
• Updates HTML lang attribute from "en" to "ja"
• Adds translation source revision comments to maintain traceability

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
content/patterns/checkbox/checkbox-pattern.html	Translates checkbox pattern guidance to Japanese
content/patterns/checkbox/examples/checkbox.html	Translates two-state checkbox example to Japanese
content/patterns/checkbox/examples/checkbox-mixed.html	Translates mixed-state checkbox example to Japanese
content/about/coverage-and-quality/role-coverage.csv	Updates CSV entries with Japanese example titles
content/about/coverage-and-quality/prop-coverage.csv	Updates CSV entries with Japanese example titles
content/about/coverage-and-quality/coverage-and-quality-report.html	Updates HTML report with Japanese example titles and removes HC indicators

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