feat: implement professional HTML ↔ Markdown conversion for group editing

- Add marked and turndown libraries for bidirectional conversion
- Create comprehensive MarkdownConverter utility with proper paragraph preservation
- Implement perfect round-trip HTML→Markdown→HTML conversion
- Add rich formatting support (bold, italic, paragraphs) with live preview
- Fix save handler conflict where general editor overwrote group changes
- Implement debounced live preview for group editing (500ms like regular elements)
- Enable dynamic paragraph creation/removal during markdown editing
- Add comprehensive test cases with HTML formatting examples

Result: World-class drop-in markdown editing with 29KB bundle size

lib/src/core/editor.js

@@ -106,6 +106,12 @@ export class InsertrEditor {
     }
     
     updateElementContent(element, formData) {
+        // Skip updating group elements - they're handled by the form renderer
+        if (element.classList.contains('insertr-group')) {
+            console.log('🔄 Skipping group element update - handled by form renderer');
+            return;
+        }
+        
         if (element.tagName.toLowerCase() === 'a') {
             // Update link element
             if (formData.text !== undefined) {