Consolidate type definitions and fix API contract

- Move all ContentItem, ContentClient, ContentResponse types to engine/types.go as single source of truth
- Remove duplicate type definitions from content/types.go
- Update all imports across codebase to use engine types
- Enhance engine to extract existing data-content-id from HTML markup
- Simplify frontend to always send html_markup, let server handle ID extraction/generation
- Fix contentId reference errors in frontend error handling
- Add getAttribute helper method to engine for ID extraction
- Add GetAllContent method to engine.DatabaseClient
- Update enhancer to use engine.ContentClient interface
- All builds and API endpoints verified working

This resolves the 400 Bad Request errors and creates a unified architecture where the server is the single source of truth for all ID generation and content type management.

lib/src/core/api-client.js

@@ -29,24 +29,15 @@ export class ApiClient {
     }
     
 
-    async createContent(contentId, content, type, htmlMarkup = null) {
+    async createContent(content, type, htmlMarkup) {
         try {
             const payload = {
+                html_markup: htmlMarkup,  // Always send HTML markup - server extracts ID or generates new one
                 value: content,
                 type: type,
                 file_path: this.getCurrentFilePath()  // Always include file path for consistent ID generation
             };
             
-            if (contentId) {
-                // Enhanced site - provide existing ID
-                payload.id = contentId;
-            } else if (htmlMarkup) {
-                // Non-enhanced site - provide HTML markup for unified engine ID generation
-                payload.html_markup = htmlMarkup;
-            } else {
-                throw new Error('Either contentId or htmlMarkup must be provided');
-            }
-            
             const response = await fetch(`${this.baseUrl}?site_id=${this.siteId}`, {
                 method: 'POST',
                 headers: {
@@ -61,7 +52,7 @@ export class ApiClient {
                 console.log(`✅ Content created: ${result.id} (${result.type})`);
                 return result;
             } else {
-                console.warn(`⚠️ Create failed (${response.status}): ${contentId || 'backend-generated'}`);
+                console.warn(`⚠️ Create failed (${response.status}): server will generate ID`);
                 return null;
             }
         } catch (error) {
@@ -69,7 +60,7 @@ export class ApiClient {
                 console.warn(`🔌 API Server not reachable at ${this.baseUrl}`);
                 console.warn('💡 Start full-stack development: just dev');
             } else {
-                console.error('Failed to create content:', contentId, error);
+                console.error('Failed to create content:', error);
             }
             return false;
         }