fix: prevent section jumping by preserving insertion position

Fixed critical positioning issue where split sections would jump to end
of document instead of staying at their original location.

Problem:
- appendChild() was adding new sections at end of parent container
- Split sections appeared at bottom of document, not at edit location
- Disrupted document flow and user experience

Solution:
- Remember original position with nextSibling before removal
- Use insertBefore(wrapper, nextSibling) for correct positioning
- New sections now appear exactly where original section was located
- Maintains proper document order and reading flow

This ensures that when you split a paragraph by adding empty lines,
the resulting sections stay in their logical position within the
document structure.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

markitect/document_manager.py

@@ -748,11 +748,12 @@ class DocumentManager:
                         // Multiple paragraphs - create separate sections
                         const parentNode = section.parentNode;
                         const sectionIndex = section.getAttribute('data-section');
+                        const nextSibling = section.nextSibling; // Remember position
 
                         // Remove the original section
                         parentNode.removeChild(section);
 
-                        // Create separate sections for each paragraph
+                        // Create separate sections for each paragraph and insert at correct position
                         paragraphs.forEach((paragraph, index) => {
                             const wrapper = document.createElement('div');
                             wrapper.innerHTML = marked.parse(paragraph.trim());
@@ -760,8 +761,8 @@ class DocumentManager:
                             wrapper.setAttribute('data-section', sectionIndex + '_' + index);
                             wrapper.setAttribute('data-edited', 'true');
 
-                            // Insert each new section
-                            parentNode.appendChild(wrapper);
+                            // Insert at the correct position (before nextSibling)
+                            parentNode.insertBefore(wrapper, nextSibling);
                         });
                     } else {
                         // Single content block - create one wrapper