feat: delete song from disk via edit-metadata modal

- /api/delete-song endpoint: resolves path via same fallback chain as
  edit-metadata, validates file is inside music root + is an audio file,
  deletes it, then prunes the album folder if empty and the artist folder
  if that too becomes empty — no recursive rm, depth strictly limited to
  two levels above the file so no accidental mass-deletes
- Safety: path.resolve() guard ensures no path-traversal outside music root;
  rmdirSync (non-recursive) fails if dir is non-empty as extra safety net
- Frontend: delete button (red outline, trash icon) at bottom of edit mode
  with confirmation dialog showing title + folder-cleanup warning

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

index.html

@@ -961,6 +961,12 @@
         <button class="info-close" style="flex:1;margin:0" onclick="toggleInfoEdit()">Abbrechen</button>
         <button class="info-close" style="flex:1;margin:0;background:var(--accent);color:#fff" onclick="saveMetadata()">Speichern</button>
       </div>
+      <div style="padding:0 20px 20px">
+        <button class="info-close" style="width:100%;margin:0;background:none;border:1px solid #c0392b;color:#c0392b;display:flex;align-items:center;justify-content:center;gap:6px" onclick="deleteSong()">
+          <svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><polyline points="3 6 5 6 21 6"/><path d="M19 6l-1 14a2 2 0 0 1-2 2H8a2 2 0 0 1-2-2L5 6"/><path d="M10 11v6"/><path d="M14 11v6"/><path d="M9 6V4h6v2"/></svg>
+          Song löschen
+        </button>
+      </div>
     </div>
   </div>
 </div>