seo: dynamic per-route title/meta description + noindex on app routes

Problem

index.html sets one static <title> and meta description shared by every route — /, /app, /app/blocks/world/:worldId/:lessonNumber, /app/thinking/world/:worldId/:lessonNumber, etc. Consequences:

• The browser tab title never changes when navigating within the app (every screen already renders its own <h1> per .ai/agents/context.md's route table, but the document <title> doesn't follow it).
• Gameplay routes inherit the landing page's marketing description in any social share preview or search result, which is inaccurate and, because lesson/world screens are highly repetitive in structure, a thin/duplicate-content risk if they were ever indexed.

Task

• Add lightweight per-route document head management — a small custom hook (e.g. useDocumentHead(title, options)) that sets document.title and updates/creates the <meta name="description"> and <meta name="robots"> tags on route change. No new dependency should be needed for this; if one is proposed anyway it must be called out in the plan per the "no new dependency without it appearing in the plan" hard constraint.
• / keeps the existing rich title/description and index, follow.
• All /app/* routes get a route-appropriate title (e.g. "{World name} — CodeKids", "{Lesson name} — CodeKids") and noindex, follow (keep follow so link equity still flows, just don't index the gameplay screens themselves).

Files

• index.html (baseline meta stays as the / default)
• src/App.tsx (wire the hook into each route)
• new hook/util, e.g. src/hooks/useDocumentHead.ts

Part of

#74

Read index.html and the route table in .ai/agents/context.md, then inspect src/App.tsx to see how the listed routes are rendered. Add the new hook or utility under src/hooks and wire it to the routes; done means / retains its existing metadata while every /app/* route has an appropriate title, description, and noindex, follow robots value.