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/features/housekeeping/foundation/preview-route-contract.test.ts

@@ -41,6 +41,8 @@ const routeMocks = vi.hoisted(() => {
 			}
 			return translate;
 		}),
+		getLocale: vi.fn(async () => "en"),
+		getMessages: vi.fn(async () => ({})),
 		notFound: vi.fn((): never => {
 			throw new Error("NEXT_NOT_FOUND");
 		}),
@@ -58,6 +60,8 @@ vi.mock("next/navigation", () => ({
 }));
 vi.mock("next-intl/server", () => ({
 	getTranslations: routeMocks.getTranslations,
+	getLocale: routeMocks.getLocale,
+	getMessages: routeMocks.getMessages,
 }));
 vi.mock("@/features/housekeeping/foundation/server-capability-context", () => ({
 	getHousekeepingCapabilityContext: routeMocks.getHousekeepingCapabilityContext,