feat: artist separator normalization + separator hints in metadata editor

- Normalize •, ·, ;, , separators on save (both edit and download paths)
- Fix ndSong scope bug (was inaccessible outside try block in edit-metadata)
- Apply _normalizeArtists to artist/albumArtist before ffmpeg tags and file rename
- Storage format: Artist A;Artist B (no space, Navidrome/MusicBrainz style)
- Display format: Artist A; Artist B (space, via _fmtArtists in frontend)
- Show separator hint below label in metadata editor fields
- Remove hover effect on player bar song title

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

backend/novasonic-server.js

@@ -240,6 +240,13 @@ app.post('/api/spotify/resolve', async (req, res) => {
   }
 });
 
+// Normalize user-supplied artist strings — accepts •, ·, ;, or , as separators
+// and rejoins with the canonical sep ("; " for track artist, ", " for album artist).
+function _normalizeArtists(str, sep) {
+  if (!str) return str;
+  return str.split(/\s*[•·;,]\s*/).map(s => s.trim()).filter(Boolean).join(sep);
+}
+
 // Strip " - Topic" suffix YouTube Music appends to artist channel names
 function _cleanYtArtist(name) {
   return (name || '').replace(/\s*-\s*Topic$/i, '').trim();
@@ -596,15 +603,20 @@ app.post('/api/edit-metadata', async (req, res) => {
           discNum, discTotal, genre, artworkBase64, artworkMime, artworkUrl: newArtworkUrl } = req.body;
   if (!ndUrl || !user || !pass || !songId) return res.status(400).json({ error: 'ndUrl, user, pass, songId required' });
 
+  // Normalize artist separators — accept •, ·, ;, , and rejoin canonically
+  const normalArtist      = _normalizeArtists(artist, ';');
+  const normalAlbumArtist = _normalizeArtists(albumArtist, ', ');
+
   // Resolve file path from Navidrome
   let filePath;
+  let ndSong;
   try {
     const salt  = Math.random().toString(36).slice(2, 10);
     const token = crypto.createHash('md5').update(pass + salt).digest('hex');
     const ndApi  = `${ndUrl}/rest/getSong?id=${songId}&u=${encodeURIComponent(user)}&t=${token}&s=${salt}&v=1.16.1&c=novasonic&f=json`;
     const ndRes  = await fetch(ndApi, { signal: AbortSignal.timeout(8000) });
     const ndData = await ndRes.json();
-    const ndSong = ndData?.['subsonic-response']?.song;
+    ndSong = ndData?.['subsonic-response']?.song;
     filePath = ndSong?.path;
     if (!filePath) return res.status(404).json({ error: 'Song not found in Navidrome' });
   } catch (e) {
@@ -659,10 +671,10 @@ app.post('/api/edit-metadata', async (req, res) => {
         ffArgs.push('-map', '0:v?'); // '?' = optional, no error if file has no cover
       }
       ffArgs.push('-c', 'copy', '-map_metadata', '-1', '-id3v2_version', '3');
-      if (title)       ffArgs.push('-metadata', `title=${title}`);
-      if (artist)      ffArgs.push('-metadata', `artist=${artist}`);
-      const resolvedAlbumArtist = albumArtist || (artist ? artist.split(';')[0].trim() : null);
-      if (resolvedAlbumArtist) ffArgs.push('-metadata', `album_artist=${resolvedAlbumArtist}`);
+      if (title)            ffArgs.push('-metadata', `title=${title}`);
+      if (normalArtist)     ffArgs.push('-metadata', `artist=${normalArtist}`);
+      const resolvedAlbumArtist = normalAlbumArtist || (normalArtist ? normalArtist.split(';')[0].trim() : null);
+      if (resolvedAlbumArtist)  ffArgs.push('-metadata', `album_artist=${resolvedAlbumArtist}`);
       if (album)       ffArgs.push('-metadata', `album=${album}`);
       if (year)        ffArgs.push('-metadata', `date=${year}`);
       if (trackNum)    ffArgs.push('-metadata', `track=${totalTracks ? `${trackNum}/${totalTracks}` : String(trackNum)}`);
@@ -691,7 +703,7 @@ app.post('/api/edit-metadata', async (req, res) => {
     // 1. Rename file if title changed — convention: {01 - }?{Artist} - {Title}.ext
     if (title) {
       const safeTitle      = sanitize(title);
-      const fileArtist     = (artist || ndSong?.artist || '').replace(/;/g, ',');
+      const fileArtist     = (normalArtist || ndSong?.artist || '').replace(/;/g, ',');
       const safeArtistPart = fileArtist ? sanitize(fileArtist) + ' - ' : '';
       const prefix         = trackNum ? String(trackNum).padStart(2, '0') + ' - ' : '';
       const newName        = `${prefix}${safeArtistPart}${safeTitle}${fileExt}`;
@@ -705,8 +717,8 @@ app.post('/api/edit-metadata', async (req, res) => {
     // 2. Move to new artist/album folder when albumArtist or album changed
     const oldAlbumDir  = path.dirname(currentFile);
     const oldArtistDir = path.dirname(oldAlbumDir);
-    const newAlbumName  = album       ? sanitize(album) : path.basename(oldAlbumDir);
-    const newArtistName = albumArtist ? sanitize(albumArtist.split(/[,;]/)[0].trim()) : path.basename(oldArtistDir);
+    const newAlbumName  = album             ? sanitize(album) : path.basename(oldAlbumDir);
+    const newArtistName = normalAlbumArtist ? sanitize(normalAlbumArtist.split(/[,;]/)[0].trim()) : path.basename(oldArtistDir);
     const newDir = path.join(CONFIG.musicPath, newArtistName, newAlbumName);
 
     if (newDir !== oldAlbumDir) {