Fix site_id isolation for demo sites

- Auto-derive site_id from demo directory paths (demos/demo-site -> 'demo', demos/simple/test-simple -> 'simple')
- Add validation requiring explicit site_id for non-demo paths with helpful error messages
- Remove JavaScript 'demo' fallback and add proper error messaging for missing site_id
- Ensure each demo site uses isolated content namespace to prevent content mixing

Resolves issue where /sites/simple and /sites/demo both used site_id=demo

lib/src/core/api-client.js

@@ -10,7 +10,7 @@ export class ApiClient {
             : '/api/content';                      // Production: same-origin API
             
         this.baseUrl = options.apiEndpoint || defaultEndpoint;
-        this.siteId = options.siteId || 'demo';
+        this.siteId = options.siteId || this.handleMissingSiteId();
         
         // Log API configuration in development
         if (isDevelopment && !options.apiEndpoint) {
@@ -276,6 +276,22 @@ export class ApiClient {
         return false;
     }
     
+    /**
+     * Handle missing site_id configuration
+     * @returns {string} Site ID or throws error
+     */
+    handleMissingSiteId() {
+        console.error('❌ No site_id configured for Insertr.');
+        console.log('💡 Add data-site-id attribute to your script tag:');
+        console.log('   <script src="insertr.js" data-site-id="mysite"></script>');
+        console.log('');
+        console.log('🚀 For demos under demos/, site_id is auto-derived from directory name.');
+        console.log('🏭 For production sites, you must explicitly set your site_id.');
+        
+        // Return a placeholder that will cause API calls to fail gracefully
+        return '__MISSING_SITE_ID__';
+    }
+
     /**
      * Get current file path from URL for consistent ID generation
      * @returns {string} File path like "index.html", "about.html"