feat: modernize documentation navigation

[Htkym]

Summary

• Enable LithoSharp 0.3.0 local search for the documentation site.
• Replace duplicate English/Japanese sidebar entries with one language-appropriate navigation set.
• Add an accessible EN/JA switcher and keep previous/next links within the selected language.
• Localize Japanese document chrome and preserve public output staging that excludes LithoSharp ownership metadata.

Validation

• Release build succeeded with zero warnings and errors.
• Fresh site generation succeeded with search.html and search-index.json.
• Japanese and English navigation, lang attributes, language switchers, and same-language pagination were smoke-tested.

[Copilot]

🟡 Changes recommended

The language switcher logic currently behaves incorrectly on pages without a real EN/JA pair (e.g., it can navigate away unexpectedly and/or misapply aria-current).

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates the LithoSharp-based documentation site generator to enable local search and to provide a cleaner, language-aware documentation UI (EN/JA), including localized chrome, a language switcher, and same-language pagination for key pages.

Changes:

• Switched the site generator to a custom PlotreeDocsTemplate wrapper around DocsSiteTemplate with EnableSearch = true.
• Replaced the default docs sidebar with a language-specific navigation set and localized key chrome strings for Japanese pages.
• Injected an EN/JA switcher into the docs header and rewrote pagination for the manual/privacy pages to stay within the selected language.

File summaries

File	Description
tools/Plotree.Site/Program.cs	Uses the new template and adds CSS for the language switcher UI.
tools/Plotree.Site/PlotreeDocsTemplate.cs	Implements HTML post-processing for language-aware sidebar, header switcher injection, JP chrome localization, and same-language pagination.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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[Htkym]

レビュー指摘を反映しました。GetLanguagePair を nullable に変更し、翻訳ペアが存在するマニュアル／プライバシーポリシーの4ページだけ EN/JA スイッチャーを表示します。トップページ、検索、サポートページでは表示せず、意図しないページ遷移と �ria-current の誤判定を防ぎます。Release build と生成後の paired/unpaired ページのスモークチェックも成功しています。