feat(housekeeping): add per-hotel theme manager with import/export and background

Theme Manager under /admin-next/hotel/theme-manager lets the owner save, apply, rename, delete, import, and export custom themes, plus set a custom site background by URL or upload. Themes are stored in WebsiteSetting/custom_themes JSON so they survive CMS updates.

src/features/housekeeping/foundation/foundation-source-contract.test.ts

@@ -580,6 +580,11 @@ describe("housekeeping foundation completion contracts", () => {
 		expect(
 			registry.domains.find((domain) => domain.id === "hotel")?.routes,
 		).toEqual([
+			expect.objectContaining({
+				id: "theme-manager",
+				href: "/admin-next/hotel/theme-manager",
+				labelKey: "pages.housekeeping.domains.hotel.routes.themeManager",
+			}),
 			expect.objectContaining({
 				id: "nitro-cleanup",
 				href: "/admin-next/hotel/nitro-cleanup",