Add three Zenn translations and improve documentation SEO

[rdlabo]

Summary

• add reviewed English translations for the Angular bundle, per-PR npm beta, and Playwright visual regression Zenn articles
• add localized intent-focused titles and descriptions for high-impression Stripe, AdMob, ESLint, and Ionic Safe Area pages
• add an ESLint legacy URL redirect, Safe Area internal links, explicit update dates, and an Ionic article-to-documentation path
• resolve related documentation URLs for both portal-hosted and externally hosted projects
• make the public docs generation entrypoint source-correct by generating article metadata before documentation
• keep page-only SEO metadata out of the eager project catalog to protect the main bundle budget

Review fixes

• derive related article publication dates from the RSS/API-generated article catalog instead of duplicating public Zenn dates in source front matter
• guarantee article-to-docs generation order for direct generation, local start, tests, and production builds
• route Ionic Docs related links to https://ionicframework.jp/docs/ instead of a nonexistent portal path

Validation

• npm run fmt:check
• npm run lint
• CI=true npm test — 140 Node, 48 docs, and 36 web-site tests
• CI=true npm run build — 626 docs routes and 164 web-site routes prerendered
• npm run seo:audit
• npm run articles:validate-translations

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[rdlabo]

This combined PR has been split by concern:

• Add three reviewed Zenn article translations #22 — reviewed Zenn article translations
• Improve documentation search visibility #23 — documentation SEO improvements

Please review and merge them independently.