seo: expand structured data (Organization/WebSite, BreadcrumbList)

[mazipan-wego]

Problem

index.html ships exactly one JSON-LD block (WebApplication). There's no Organization/WebSite entity, which limits eligibility for brand-level rich results (e.g. sitelinks searchbox), and no BreadcrumbList for the world → lesson hierarchy.

Task

• Add a complementary WebSite (and/or Organization) JSON-LD block alongside the existing WebApplication one in index.html.
• Consider BreadcrumbList structured data once per-route metadata lands (see the "dynamic per-route title" sub-issue) — sequence this one after that if it depends on route-level data being available.
• Validate the final output with Google's Rich Results Test before merging (manual verification step, note the result in the plan's implementation notes).

Files

• index.html

Part of

#74