feat: implement unified content engine to eliminate ID generation inconsistencies

- Create internal/engine module as single source of truth for content processing
- Consolidate 4 separate ID generation systems into one unified engine
- Update API handlers to use engine for consistent server-side ID generation
- Remove frontend client-side ID generation, delegate to server engine
- Ensure identical HTML markup + file path produces identical content IDs
- Resolve content persistence failures caused by ID fragmentation between manual editing and enhancement processes

lib/src/core/api-client.js

@@ -29,21 +29,22 @@ export class ApiClient {
     }
     
 
-    async createContent(contentId, content, type, elementContext = null) {
+    async createContent(contentId, content, type, htmlMarkup = null) {
         try {
             const payload = {
                 value: content,
-                type: type
+                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 (elementContext) {
-                // Non-enhanced site - provide context for backend ID generation
-                payload.element_context = elementContext;
+            } 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 elementContext must be provided');
+                throw new Error('Either contentId or htmlMarkup must be provided');
             }
             
             const response = await fetch(`${this.baseUrl}?site_id=${this.siteId}`, {
@@ -283,4 +284,17 @@ export class ApiClient {
         
         return false;
     }
+    
+    /**
+     * Get current file path from URL for consistent ID generation
+     * @returns {string} File path like "index.html", "about.html"
+     */
+    getCurrentFilePath() {
+        const path = window.location.pathname;
+        if (path === '/' || path === '') {
+            return 'index.html';
+        }
+        // Remove leading slash: "/about.html" → "about.html"
+        return path.replace(/^\//, '');
+    }
 }