refactor: add and use isValidCatalogLayout type guard for robust layout assignment

src/lib/catalog-layouts.ts

@@ -23,6 +23,11 @@ export const CATALOG_LAYOUTS = [
 
 export type CatalogLayout = (typeof CATALOG_LAYOUTS)[number];
 
+/** Type guard to check if a string is a valid CatalogLayout. */
+export function isValidCatalogLayout(layout: string): layout is CatalogLayout {
+	return (CATALOG_LAYOUTS as readonly string[]).includes(layout);
+}
+
 /** Short human-readable description for each layout. Shown in the layout selector. */
 export const LAYOUT_DESCRIPTIONS: Record<CatalogLayout, string> = {
 	default_3x3: "Standard 3-column grid of items",