perf(i18n): stop serializing the full catalogue on public pages

The root layout fed the entire ~254 KB message catalogue to
NextIntlClientProvider, which serializes it into the RSC payload of every
page. A public page only reads ~59 KB of it; the ~195 KB difference is staff
tool copy that only /admin, /admin-next and /mod ever render.

The root provider now installs the reduced catalogue and each staff route
group re-installs the full one, since a nested NextIntlClientProvider replaces
the parent message set rather than merging into it. pages.admin.actions stays
public because the shared useServerAction hook runs on public pages too.

Measured: 253 KB -> 59 KB of messages per public page (raw JSON).

Guard tests cover all three halves: the root keeps using the reduced
catalogue, every staff layout installs its own full provider, and no public
tree reads a staff-only namespace.

src/app/mod/layout.tsx

@@ -10,7 +10,8 @@ import {
 	Users,
 } from "lucide-react";
 import { redirect } from "next/navigation";
-import { getTranslations } from "next-intl/server";
+import { NextIntlClientProvider } from "next-intl";
+import { getLocale, getMessages, getTranslations } from "next-intl/server";
 import type { ReactNode } from "react";
 import { LanguageSwitcher } from "@/components/language-switcher";
 import Link from "@/components/link";
@@ -81,8 +82,13 @@ export default async function ModLayout({ children }: { children: ReactNode }) {
 		{ href: "/mod/team", label: t("nav.team"), icon: Users, show: showTeam },
 	].filter((item) => item.show);
 
+	// The root layout ships the reduced catalogue so public pages do not carry
+	// the ~195 KB of staff-tool strings. Nested providers replace the parent
+	// set rather than merging, so staff route groups re-install the full one.
+	const [locale, messages] = await Promise.all([getLocale(), getMessages()]);
+
 	return (
-		<>
+		<NextIntlClientProvider locale={locale} messages={messages}>
 			{csrfToken ? <meta name="csrf-token" content={csrfToken} /> : null}
 			<div
 				data-admin
@@ -140,6 +146,6 @@ export default async function ModLayout({ children }: { children: ReactNode }) {
 				</header>
 				<main className="mx-auto max-w-6xl p-4 sm:p-6">{children}</main>
 			</div>
-		</>
+		</NextIntlClientProvider>
 	);
 }