fix: fetch highlights by article reference instead of author

- Add fetchHighlightsForArticle() to query highlights by article coordinate
- Use #a tag filter to find highlights that reference the article
- Query well-known relays for highlights even without authentication
- Extract article's d-tag and construct coordinate (kind:pubkey:identifier)
- Keep original fetchHighlights() for fetching user's own highlights
- Add detailed logging for debugging highlight fetching

This fixes the issue where no highlights were shown because we were
querying for highlights created BY the article author rather than
highlights created ABOUT the article.

.cursor/rules/highlights-nip-and-docs.mdc

@@ -0,0 +1,3 @@
+---
+alwaysApply: true
+---