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>

i18n/request.js

@@ -0,0 +1,26 @@
+import { getRequestConfig } from "next-intl/server";
+import { hasLocale } from "next-intl";
+import { routing } from "./routing";
+
+export default getRequestConfig(async ({ requestLocale }) => {
+	let locale = await requestLocale;
+	if (!locale || !hasLocale(routing.locales, locale)) {
+		locale = routing.defaultLocale;
+	}
+
+	let messages = {};
+	try {
+		messages = (await import(`../messages/${locale}.json`)).default;
+	} catch {
+		try {
+			messages = (await import("../messages/pt.json")).default;
+		} catch {
+			messages = {};
+		}
+	}
+
+	return {
+		locale,
+		messages,
+	};
+});