build: configure Cloudflare Pages deployment

- Add wrangler CLI and workerd runtime as dev dependencies
- Configure nitro prerender with autoSubfolderIndex disabled for Pages
- Add Turnstile validation endpoint for server-side CAPTCHA verification
- Add baseline-browser-mapping for browser compatibility

.env.example

@@ -1,3 +1 @@
 NUXT_PUBLIC_BACKEND_URL=http://localhost:3100
-NUXT_PUBLIC_TURNSTILE_SITE_KEY="changeme"
-NUXT_TURNSTILE_SECRET_KEY="changeme"