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/layout.tsx

@@ -13,6 +13,7 @@ import { ThemeVars } from "@/components/theme-vars";
 import { enforceSiteAccess } from "@/lib/access-guard";
 import { getDefaultFaviconDataUri } from "@/lib/default-favicon";
 import { resolveHotelName } from "@/lib/hotel-name";
+import { publicClientMessages } from "@/lib/i18n-client-messages";
 import { siteSettings } from "@/lib/services/site-settings";
 import "./globals.css";
 
@@ -68,7 +69,11 @@ export default async function RootLayout({
 }) {
 	await enforceSiteAccess();
 	const locale = await getLocale();
-	const messages = await getMessages();
+	// Only the namespaces non-staff routes can reach: the full catalogue is
+	// ~254 KB and would otherwise be serialised into every public page's RSC
+	// payload. `/admin`, `/admin-next` and `/mod` install a nested provider
+	// seeded with the full set (see i18n-client-messages).
+	const messages = publicClientMessages(await getMessages());
 	const defaultDark = await siteSettings.getBool("default_dark", false);
 	const nitroUrl = await siteSettings.get("nitro_client_url", "");
 	const nonce = (await headers()).get("x-nonce") ?? undefined;