fix(config): ensure is_demo_mode is persisted in site.config.json and typed in SiteConfig interface

src/utils/configLoader.ts

@@ -73,6 +73,7 @@ export interface SiteConfig {
   sectionsOrder: string[];
 
   sectionsDetails?: SectionConfig[];
+  is_demo_mode?: boolean;
   site_info?: {
     title?: string;
     company_name?: string;
@@ -117,6 +118,8 @@ export interface SmtpConfig {
 export const FALLBACK_SITE_CONFIG: SiteConfig = {
   siteName: 'N&D IT Solutions',
   metaDescription: 'High-Performance Webdesign & Digital Solutions - 100% DSGVO-konform.',
+  is_demo_mode: false,
+
   header: {
     brandName: 'N&D',
     brandHighlight: 'i-t SOLUTIONS',