list-editor: add per-track description button + modal

Each music list row now shows a 설명 button immediately to the left of
the 별칭 button. Click opens a modal with a multi-line textarea; on
close the value is persisted into MusicListEntry.description and saved
to the same pack list JSON. The button gets a hasDesc visual indicator
when filled. Description is stored but intentionally not consumed by
datapack export or alias matching — purely informational metadata.

- types.ts: add description: string to MusicListEntry
- store.ts: normalize entry.description via sanitizeStr (defaults to '')
- listEditor.ejs: new #descModal alongside aliasModal
- listEditor.js: render descBtn left of aliasBtn, attach handlers,
  also set description: '' on playlist-fetched entries
- styles.css: extend trackRow grid to 6 cols, reuse aliasBtn styling
  for descBtn, add descTextarea sizing
- locale (ko-kr): descBtn / descModalTitle / descBack / descPlaceholder
  / descHint

Backwards-compatible: existing list JSON files without description
field normalize to ''.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

views/op/listEditor.ejs

@@ -124,6 +124,21 @@
     </div>
   </div>
 
+  <!-- Description modal (music) -->
+  <div class="modalOverlay" id="descModal" hidden>
+    <div class="modalCard">
+      <header class="aliasModalHeader">
+        <button type="button" class="ghostLink" id="desc-back"><%= t('listEditor.descBack') %></button>
+        <h3 id="desc-modal-title"></h3>
+        <span></span>
+      </header>
+      <div class="modalBody">
+        <p class="muted" style="margin:0;font-size:12px;"><%= t('listEditor.descHint') %></p>
+        <textarea id="desc-textarea" class="textInput descTextarea" placeholder="<%= t('listEditor.descPlaceholder') %>" rows="6"></textarea>
+      </div>
+    </div>
+  </div>
+
   <!-- Edit modal (image) -->
   <div class="modalOverlay" id="editImageModal" hidden>
     <div class="modalCard">