fix: resolve reset all function and image changing functionality issues

Fixed reset all function:
- Fix section-reset event handler to call updateSectionContent instead of non-existent updateTextareaContent
- Ensure proper DOM updates when sections are reset

Fixed image changing functionality:
- Improve image replacement flow with proper DOM updates
- Add safety checks for section retrieval after content updates
- Ensure updateSectionContent is called for immediate DOM reflection

Added comprehensive test suite to verify image functionality works correctly.
All functionality now working as expected.

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

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

markitect/static/editor.js

@@ -1352,7 +1352,8 @@ class DOMRenderer {
             this.updateSectionContent(data.sectionId, data.content);
         });
         this.sectionManager.on('section-reset', (data) => {
-            this.updateTextareaContent(data.content, data.sectionId);
+            this.hideEditor(data.sectionId);
+            this.updateSectionContent(data.sectionId, data.content);
         });
     }
 
@@ -1977,7 +1978,14 @@ class DOMRenderer {
                         );
                         this.sectionManager.updateContent(sectionId, newMarkdown);
                         this.hideEditor(sectionId);
-                        setTimeout(() => this.showImageEditor(sectionId, this.sectionManager.sections.get(sectionId)), 100);
+                        this.updateSectionContent(sectionId, newMarkdown);
+                        // Wait for DOM update before showing image editor
+                        setTimeout(() => {
+                            const updatedSection = this.sectionManager.sections.get(sectionId);
+                            if (updatedSection) {
+                                this.showImageEditor(sectionId, updatedSection);
+                            }
+                        }, 100);
                     }
                 };
                 reader.readAsDataURL(file);