refactor: use applesauce helpers for highlight parsing

- Replace manual tag parsing with applesauce-core helper functions
- Use getHighlightText, getHighlightContext, getHighlightComment, etc.
- Add support for highlight comments in UI
- Extract author from attributions using proper helper
- Handle both event and address pointers correctly
- Add styling for highlight comments

This follows applesauce best practices and makes the code more robust.

src/components/HighlightItem.tsx

@@ -37,6 +37,12 @@ export const HighlightItem: React.FC<HighlightItemProps> = ({ highlight, onSelec
           {highlight.content}
         </blockquote>
         
+        {highlight.comment && (
+          <div className="highlight-comment">
+            {highlight.comment}
+          </div>
+        )}
+        
         {highlight.context && (
           <details className="highlight-context">
             <summary>Show context</summary>