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/lib/theme-custom-store.ts

@@ -1,5 +1,6 @@
 import { db, WebsiteSetting } from "@/lib/db";
 import { siteSettings } from "@/lib/services/site-settings";
+import { BACKGROUND_DEFAULTS, BACKGROUND_KEYS } from "@/lib/theme-bg";
 import {
 	PRESETS,
 	THEME_COLOR_KEYS,
@@ -29,6 +30,7 @@ const EXTRA_KEYS = [
 	"size_heading_h3",
 	"custom_css",
 	"theme_preset",
+	...BACKGROUND_KEYS,
 ];
 
 const ALL_KEYS: string[] = [
@@ -59,8 +61,9 @@ function fallbackFor(key: string): string {
 		size_heading_h3: "18",
 		custom_css: "",
 		theme_preset: "Atom (golden)",
+		...BACKGROUND_DEFAULTS,
 	};
-	return extras[key] ?? ""; // eslint-disable-line security/detect-object-injection -- key derived from internal THEME_COLOR_KEYS
+	return extras[key] ?? ""; // eslint-disable-line security/detect-object-injection -- key derived from internal EXTRA_KEYS
 }
 
 export async function listCustomThemes(): Promise<CustomTheme[]> {