refactor: Complete UI cleanup and simplify editor architecture

- Remove complex style preservation system from editor
- Simplify markdown conversion back to straightforward approach
- Remove StyleContext class and style-aware conversion methods
- Switch content type from 'html' back to 'markdown' for consistency
- Clean up editor workflow to focus on core markdown editing
- Remove ~500 lines of unnecessary style complexity

This completes the UI unification cleanup by removing the overly complex
style preservation system that was making the editor harder to maintain.

internal/engine/injector.go

@@ -103,8 +103,6 @@ func (i *Injector) InjectBulkContent(elements []ElementWithID) error {
 			i.injectTextContent(elem.Element.Node, contentItem.Value)
 		case "markdown":
 			i.injectMarkdownContent(elem.Element.Node, contentItem.Value)
-		case "html":
-			i.injectHTMLContent(elem.Element.Node, contentItem.Value)
 		case "link":
 			i.injectLinkContent(elem.Element.Node, contentItem.Value)
 		default: