Add genre data for [+] expansion feature

- Populate lineup.json with genres for 370/569 artists (65% coverage)
- Use Last.fm API as primary source (231 artists), MusicBrainz as fallback (32 artists)
- Update add-genres.js script to try Last.fm first (faster, better coverage)
- Fix expansion message from placeholder to proper "No similar artists" text
- Add dotenv dependency for API key management

Genre expansion now shows other Treefort artists with overlapping genres
when clicking [+] on a matched artist.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: compunet-jimmy

package-lock.json

@@ -9,6 +9,7 @@
     "preview": "vite preview"
   },
   "devDependencies": {
+    "dotenv": "^17.3.1",
     "vite": "^5.4.0"
   }
 }