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>

index.html

@@ -654,12 +654,10 @@
           <span>Abspielhäufigkeit anzeigen</span>
           <div id="s-show-playcount" class="toggle" onclick="this.classList.toggle('on')"></div>
         </div>
-        <div class="hint" style="margin-top:6px">Zeigt Wiedergabezahl</div>
         <div class="edit-pl-toggle-row" style="margin-top:12px">
           <span>Romaji anzeigen</span>
           <div id="s-show-romaji" class="toggle" onclick="this.classList.toggle('on')"></div>
         </div>
-        <div class="hint" style="margin-top:6px">Zeigt japanische Titel und Künstlernamen in Romaji</div>
       </div>
       <div class="settings-section">
         <h3 style="display:flex;align-items:center;gap:8px;margin-bottom:12px">