fix: improve yt-dlp search reliability and artist folder structure

- Switch to YouTube Music search (ytmsearch3) as primary source with
  fallback to ytsearch5 if no result found
- Add duration filter (±30s/+45s) and live-stream exclusion via
  --match-filter when iTunes trackTimeMillis is available
- Send trackTimeMillis from frontend in download payload
- Pass durationMs through downloadTrack and downloadAlbumTracks
- Use first artist only (split on comma) for folder path on multi-artist tracks

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

src/ui.js

@@ -294,10 +294,11 @@ export async function executeDownload() {
     itunesId:    S.downloadTarget.trackId || S.downloadTarget.collectionId || null,
     collectionId: S.downloadTarget.collectionId || null,
     explicit:    S.downloadTarget.trackExplicitness === 'explicit',
-    trackNum:    S.downloadTarget.trackNumber  || undefined,
-    totalTracks: S.downloadTarget.trackCount   || undefined,
-    discNum:     S.downloadTarget.discNumber   || undefined,
-    discTotal:   S.downloadTarget.discCount    || undefined,
+    trackNum:    S.downloadTarget.trackNumber    || undefined,
+    totalTracks: S.downloadTarget.trackCount     || undefined,
+    discNum:     S.downloadTarget.discNumber     || undefined,
+    discTotal:   S.downloadTarget.discCount      || undefined,
+    durationMs:  S.downloadTarget.trackTimeMillis || undefined,
   };
   addJob(jobId, payload.title, payload.artist, payload.type);
   try {