feat: enable markdown processing for span elements

Backend changes:
- Updated parser to treat <span> elements as markdown by default
- Changed span content type detection from ContentText to ContentMarkdown
- Spans now support **bold**, *italic*, and [links](url) formatting

Frontend changes:
- Updated content type detection to match backend behavior
- Frontend now treats spans as markdown elements for consistent processing
- Editor preview behavior now matches server-side enhancement

Benefits:
- <span class='highlight'>**bold** text</span> now processes markdown correctly
- Inline elements maintain all styling while supporting rich formatting
- Complete frontend/backend consistency for span element handling
- Expands markdown support to common inline wrapper elements

Tested and verified:
- Span elements preserve all classes, IDs, and styles
- Bold, italic, and link formatting works in span elements
- Content type properly detected as 'markdown' in both systems

lib/src/core/insertr.js

@@ -339,6 +339,8 @@ export class InsertrCore {
                 return 'link';
             case 'div': case 'section':
                 return 'markdown';
+            case 'span':
+                return 'markdown'; // Match backend: spans support inline markdown
             default:
                 return 'text';
         }