[v1.0.8] Fix Google indexing and clear vercel-seo-audit findings

[tatsuzo-arkstudios]

Summary

Default Japanese pages were being served at /ja/ while the site root / had no real content, so Google had nothing canonical to index for arkstudios.co.jp. This PR moves the Japanese home to the root, adds proper hreflang/canonical wiring with x-default, and ships supporting fixes (sitemap, robots, structured data, defer-on-noModule script). Also bundles four rounds of dependabot bumps that piled up on develop.

What changed

• src/app/page.tsx: new root / page that renders the Japanese home directly, with its own generateMetadata and a canonical pointing at /. The default locale is no longer a redirect target — it's the indexable URL.
• src/app/ui/components/home-page.tsx: extracted the home view (canvas + business sections) so both / and /ja/ render the same component. Adds an Organization JSON-LD block with legalName, logo, sameAs, and inLanguage. Also sets explicit width/height on the hero <img> to stop CLS.
• src/app/[language]/(overview)/page.tsx: thinned down to a wrapper around <HomePage />. The duplicated section components were deleted from here.
• src/app/i18n/urls.ts: localePath now returns / for the default language instead of /ja/. buildLanguageAlternates adds x-default so Google has an explicit fallback for unmatched locales.
• src/app/sitemap.ts: root entry for ja is / (not /ja/), and every entry gets the x-default alternate. No URL is ever advertised in two canonical forms.
• src/app/metadata.ts: new shared module. sharedMetadata holds the icons / metadataBase / msapplication tags that used to live in [language]/layout.tsx. localizedMetadata({ description, language, url? }) returns the OG/Twitter/title block. The per-locale layout calls into it instead of redefining everything.
• src/app/layout.tsx: uses sharedMetadata, and adds a preconnect to static.cloudflareinsights.com so Cloudflare Web Analytics opens the connection during HTML parse.
• src/app/[language]/layout.tsx: collapsed from ~60 lines of metadata to a single localizedMetadata(...) call.
• src/app/not-found.tsx: the home button now points at / (was /ja/), and both buttons use next/link for client-side nav.
• src/app/ui/components/navigation-bar-client.tsx: introduces homePath / localizedPath so the logo, language toggle, and current-path highlighting all agree that the default locale lives at /. Also moves the i18n import above the config.autoAddCss = false line so ESLint's import-order rule stops complaining.
• public/robots.txt: new file. Allow: / plus a Sitemap: line pointing at https://arkstudios.co.jp/sitemap.xml.
• scripts/postprocess-export.mjs + package.json build script: post-export pass that adds defer to Next's noModule polyfill <script> in every emitted HTML file. Audits flag it as render-blocking otherwise. The regex only touches scripts that have noModule and don't already have async / defer / type="module".
• .github/workflows/ci.yml: build step now runs npm run build instead of npx --no-install next build, so the postprocess script actually runs in CI.
• Dependency bumps merged from dependabot: Next 16.2.1 → 16.2.4 (plus matching eslint-config-next), TypeScript 5 → 6.0.3, eslint 9 pinned, and the usual @typescript-eslint / autoprefixer / postcss / prettier point releases. No behavior changes.

Test plan

• npm run lint
• npm run build — out/ contains a real index.html at the root, and head scripts have defer
• Spot-checked out/sitemap.xml — / is the ja entry, every URL has x-default
• Re-run npx vercel-seo-audit https://bitremote.app after deploy to confirm production picks up the fixes