fix: match-filter trailing-s optional + playlist search fallback for special chars

- buildTitleKeywords: append '?' to keywords ending in 's' so regex
  "Whispers?" matches both "Whisper" (YouTube title) and "Whispers"
  (iTunes title) — fixes NoAki Whispers downloading wrong track
- _createNdPlaylist: if artist+title search returns no title match,
  retry with title-only query so tracks with special chars in artist
  name (e.g. "Kløn") are still found in Navidrome

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

backend/novasonic-server.js

@@ -83,7 +83,9 @@ function buildTitleKeywords(title) {
     .filter(w => w.length >= 3 && !VARIANT_WORDS.has(w.toLowerCase()) && !STOPWORDS.has(w.toLowerCase()));
   const main = words.sort((a, b) => b.length - a.length)[0] || '';
   const qualifier = lower.includes('mix') ? MIX_QUALIFIERS.find(q => lower.includes(q)) : null;
-  const result = main ? [main] : [];
+  // Make trailing 's' optional so "Whispers?" matches both "Whisper" and "Whispers" in yt-dlp regex
+  const toPattern = w => (w.length >= 4 && w.endsWith('s')) ? w + '?' : w;
+  const result = main ? [toPattern(main)] : [];
   if (qualifier && main.toLowerCase() !== qualifier) result.push(qualifier);
   return result;
 }