fix: normalize diacritics in playlist artist matching

Use Unicode property escapes to strip only symbols (☆, !, etc.)
while keeping Unicode letters (ø, ä, é) — fixes "Kløn" becoming
"Kln" instead of staying "Kløn" in the search query.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/novasonic-server.js

@@ -242,8 +242,8 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   }
 
   const outputTemplate = path.join(dir, `${trackPrefix}${safeTitle}.%(ext)s`);
-  // Strip non-ASCII chars (e.g. ☆) from artist name for cleaner search ranking
-  const searchArtist = mainArtist.replace(/[^\x00-\x7F]/g, '').trim();
+  // Strip symbol chars (e.g. ☆) but keep Unicode letters like ø, ä, é
+  const searchArtist = mainArtist.replace(/[^\p{L}\p{N}\s,&\-]/gu, '').trim();
   const query = `${searchArtist} - ${title}`;
 
   // Step 1: yt-dlp — audio only, no thumbnail (we embed iTunes artwork ourselves)