Host the example on Cloudflare Pages: embed mode, CI route checks

[pietechsolution]

Prepares the example to run as a hosted demo on Cloudflare Pages and to be embedded in documentation pages.

Changes

• ?embed=1. Set once in src/App.tsx: the layout route reads the query string, drops the navigation and adds an embed class that hides the route heading. Works on every route.
• Cloudflare Pages. No config file is needed. The routes are client side (React Router BrowserRouter), and Pages already serves index.html for any path that is not a file when the build has no top level 404.html. A public/_redirects with /* /index.html 200 was tried and dropped: Wrangler rejects it as an infinite loop rule and ignores it. Build command npm run build, output directory dist, VITE_ variables set as build variables. Documented in a new "Hosting on Cloudflare" README section at the end.
• CI route checks. CI still builds every push. When the DEMO_EVENT_KEY and DEMO_PUBLIC_KEY repository secrets exist (optional DEMO_SEASON_KEY), it serves the build with npm run preview and runs scripts/check-routes.mjs, which loads every route with and without ?embed=1 in Chromium and fails on an HTTP error, an uncaught exception, a console error, or a visible navigation in embed mode. Forks without the secrets only build.
• Empty optional event keys. An empty VITE_SEATLAYER_EVENT_KEY_2 / _3 (as in a copied .env.example) now falls back to the main key instead of producing an empty key.
• The events list is client side configuration with no server, so there is no account-wide listing to change.

Verification

• npm ci && npm run build.
• vite preview without keys: scripts/check-routes.mjs passes on all 12 route and embed combinations (setup notices, no console errors, navigation hidden with ?embed=1).
• wrangler pages dev dist: /, /seat-picker, /events?embed=1, /control-room and an unknown path all return index.html with 200, and the route check passes against it.

Nothing was deployed.

Held: merge when the hosted demo is ready.