feat: implement single POST upsert API with automatic ID generation

- Replace separate POST/PUT endpoints with unified POST upsert
- Add automatic content ID generation from element context when no ID provided
- Implement version history preservation before content updates
- Add element context support for backend ID generation
- Update frontend to use single endpoint for all content operations
- Enhanced demo site with latest database content including proper content IDs

lib/src/core/api-client.js

@@ -28,36 +28,7 @@ export class ApiClient {
         }
     }
     
-    async updateContent(contentId, content) {
-        try {
-            const response = await fetch(`${this.baseUrl}/${contentId}?site_id=${this.siteId}`, {
-                method: 'PUT',
-                headers: {
-                    'Content-Type': 'application/json',
-                    'Authorization': `Bearer ${this.getAuthToken()}`
-                },
-                body: JSON.stringify({ value: content })
-            });
-            
-            if (response.ok) {
-                console.log(`✅ Content updated: ${contentId}`);
-                return true;
-            } else {
-                console.warn(`⚠️ Update failed (${response.status}): ${contentId}`);
-                return false;
-            }
-        } catch (error) {
-            // Provide helpful error message for common development issues
-            if (error.name === 'TypeError' && error.message.includes('fetch')) {
-                console.warn(`🔌 API Server not reachable at ${this.baseUrl}`);
-                console.warn('💡 Start full-stack development: just dev');
-            } else {
-                console.error('Failed to update content:', contentId, error);
-            }
-            return false;
-        }
-    }
-    
+
     async createContent(contentId, content, type, elementContext = null) {
         try {
             const payload = {