feat: load actual file sort tags in edit modal + remove settings hints

- backend: new GET /api/song-sorttags reads TITLESORT/ARTISTSORT/ALBUMARTISTSORT/
  ALBUMSORT directly from the file via ffprobe
- ui.js: pre-fill sort fields from romaji index, then overwrite with real file
  tags from the backend (shows actual values even if manually set)
- index.html: remove redundant hint texts in settings

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

src/ui.js

@@ -356,12 +356,22 @@ export function toggleInfoEdit() {
   document.getElementById('ie-disc').value        = s.discNumber   || '';
   document.getElementById('ie-totaltracks').value = s.albumSongCount || '';
 
-  // Sort tags — pre-fill from romaji index when available
+  // Sort tags — pre-fill from romaji index, then overwrite with actual file tags if backend available
   const _rm = romajiLookup(s.id);
   document.getElementById('ie-titlesort').value        = _rm?.titleRomaji  || '';
   document.getElementById('ie-artistsort').value       = _rm?.artistRomaji || '';
   document.getElementById('ie-albumartistsort').value  = _rm?.artistRomaji || '';
   document.getElementById('ie-albumsort').value        = _rm?.albumRomaji  || '';
+  if (S.cfg.backendUrl && s.id) {
+    const p = new URLSearchParams({ ndUrl: S.cfg.url, user: S.cfg.user, pass: S.cfg.pass, songId: s.id });
+    fetch(`${S.cfg.backendUrl}/api/song-sorttags?${p}`).then(r => r.ok ? r.json() : null).then(t => {
+      if (!t) return;
+      if (t.titleSort)       document.getElementById('ie-titlesort').value       = t.titleSort;
+      if (t.artistSort)      document.getElementById('ie-artistsort').value      = t.artistSort;
+      if (t.albumArtistSort) document.getElementById('ie-albumartistsort').value = t.albumArtistSort;
+      if (t.albumSort)       document.getElementById('ie-albumsort').value       = t.albumSort;
+    }).catch(() => {});
+  }
 
   // Art preview — use existing cover
   const cv = coverUrl(s.coverArt, 200);