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>

public/styles.css

@@ -407,19 +407,24 @@ body.siteBody.centerLayout {
 .trackList { list-style: none; padding: 0; margin: 0; display: flex; flex-direction: column; gap: 4px; }
 .trackRow {
   display: grid;
-  grid-template-columns: 36px 80px 1fr auto auto;
+  grid-template-columns: 36px 80px 1fr auto auto auto;
   gap: 12px; align-items: center;
   padding: 8px 12px; background: var(--bg-card);
   border: 1px solid var(--border); border-radius: 8px;
   cursor: grab; user-select: none;
 }
-.aliasBtn {
+.aliasBtn, .descBtn {
   background: var(--bg); border: 1px solid var(--border); color: var(--text);
   padding: 6px 10px; border-radius: 6px; cursor: pointer; font-size: 12px;
   white-space: nowrap;
 }
-.aliasBtn:hover { border-color: var(--accent); }
-.aliasBtn.hasAliases { border-color: var(--accent); color: var(--accent); }
+.aliasBtn:hover, .descBtn:hover { border-color: var(--accent); }
+.aliasBtn.hasAliases, .descBtn.hasDesc { border-color: var(--accent); color: var(--accent); }
+.descTextarea {
+  width: 100%; min-height: 140px; resize: vertical;
+  font-family: inherit; font-size: 13px; line-height: 1.5;
+  padding: 8px 10px;
+}
 
 /* 별칭 모달 */
 .aliasModalHeader {