Add bilingual Anchor web demo

[Drew-Z]

What changed

• replaces the unpublished placeholder with a bilingual, source-traced static demo
• adds three datasets, twelve exercises, citations, progress recovery, and scripted tutor hints
• moves the demo under the Cloudflare Pages output and adds security/SEO assets
• updates stale rename/deployment documentation and stops tracking Wrangler account caches

Why

The production /app path previously fell back to the English landing page because the placeholder lived outside web/landing. The new demo is shipped with the product site and truthfully uses bundled browser-only data.

Validation

• npm test (5 Node contract tests, 8 Chromium flow/visual tests)
• desktop/tablet/mobile screenshots inspected
• git diff --check
• staged sensitive-pattern scan