feat(i18n): routing and localization infrastructure (phase 1)

Implement Phase 1 of i18n spec:
- Configure next-intl with 6 locales (pt, en, es, fr, de, zh-CN) and pathnames
- Move routes to app/[locale] structure with localized layouts and metadata
- Add middleware for root language negotiation (307), legacy redirects (308), and 404 for non-pt book
- Implement language selector dropdown in header with cookie persistence
- Add system typography rules for zh-CN and word-break/hyphenation for de/fr

Co-Authored-By: Antigravity <noreply@google.com>

next.config.mjs

@@ -1,3 +1,7 @@
+import createNextIntlPlugin from "next-intl/plugin";
+
+const withNextIntl = createNextIntlPlugin("./i18n/request.js");
+
 /** @type {import('next').NextConfig} */
 const nextConfig = {
 	// A imagem Docker liga isto (NEXT_OUTPUT=standalone) para empacotar só o que
@@ -13,4 +17,5 @@ const nextConfig = {
 	},
 };
 
-export default nextConfig;
+export default withNextIntl(nextConfig);
+