fix(backend): fix-artist-sort uses kanji for folder/filename, romaji only for sort tags

Consistent with point 7: original (kanji) names in folder, filename,
and main tags — romaji only in ARTISTSORT/ALBUMARTISTSORT.

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

backend/novasonic-server.js

@@ -785,7 +785,7 @@ app.post('/api/fix-artist-sort', async (req, res) => {
 
     let done = 0, errors = 0;
     const errorDetails = [];
-    const safeArtistDir = sanitize(artistSort.split(/[,;&]/)[0].trim());
+    const safeArtistDir = sanitize(artistKanji.split(/[,;&]/)[0].trim());
 
     for (const ndSong of songs) {
       try {
@@ -840,7 +840,7 @@ app.post('/api/fix-artist-sort', async (req, res) => {
         // Split on first " - " or " – ": if 2+ segments, first is old artist, rest is title
         const segments  = afterPrefix.split(/\s+[-–—]\s+/);
         const titlePart = segments.length >= 2 ? segments.slice(1).join(' - ') : afterPrefix;
-        const safeArtistFile = sanitize(artistSort.split(/[,;&]/)[0].trim());
+        const safeArtistFile = sanitize(artistKanji.split(/[,;&]/)[0].trim());
         const safeTitlePart  = sanitize(titlePart);
         const newBasename    = `${trackPrefix}${safeArtistFile} - ${safeTitlePart}${fileExt}`;