Prepare multilingual SEO and deployment readiness

[RobTar97]

What changed

• generate a crawlable public homepage, three localized hubs, 144 localized topic pages, teacher/about/privacy pages, sitemap, robots, llms.txt, canonicals, hreflang, and truthful JSON-LD from the validated topic catalog
• move the interactive learner/teacher workspace to noindexed /app/ URLs and preserve room/practice sharing
• add SEO, deployment-origin, performance, status-code, and release-readiness gates
• add route-level code splitting and document the public/app architecture and launch checklist

Why

The curriculum previously lived behind a client-rendered onboarding flow with no stable topic URLs, sitemap, canonical policy, or real 404 behavior. This prepares the open-source project for a future production domain without deploying it.

User and developer impact

Search engines can discover durable localized curriculum pages, while learners and teachers keep the existing interactive flow under /app/. Deployment remains opt-in and is blocked until a real PUBLIC_SITE_URL plus Cloudflare credentials are configured.

Validation

• npm run check
• npm audit --audit-level=high
• PUBLIC_SITE_URL=https://practice.linguaflow.dev npm run check:deploy
• PUBLIC_SITE_URL=https://practice.linguaflow.dev npx wrangler deploy --dry-run
• local Wrangler status/header checks
• desktop and responsive browser QA for /, /app/, EN/JA topic pages