feat: Implement HTML-first style preservation system

- Add StyleContext class for extracting and applying HTML attributes/styles
- Enhance MarkdownConverter with style-aware conversion methods
- Switch backend storage from markdown to HTML with 'html' content type
- Update editor workflow to preserve CSS classes, IDs, and attributes
- Maintain markdown editing UX while storing HTML for style preservation
- Support complex attributes like rel, data-*, aria-*, etc.

This enables editing styled content like <a class="fancy" rel="me">text</a>
while preserving all styling attributes through the markdown editing process.

internal/engine/injector.go

@@ -103,6 +103,8 @@ 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: