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>

locales/server/ko-kr.json

@@ -78,6 +78,11 @@
     "aliasPlaceholder": "별칭 입력",
     "aliasRemove": "삭제",
     "aliasHint": "정답으로 인정할 다른 표기·번역·약칭을 추가할 수 있습니다.",
+    "descBtn": "설명",
+    "descModalTitle": "설명 - {{title}}",
+    "descBack": "← 돌아가기",
+    "descPlaceholder": "이 곡에 대한 설명을 입력하세요",
+    "descHint": "곡 소개·트리비아 등 자유 메모. 정답 채점이나 데이터팩에는 사용되지 않습니다.",
     "metaLoading": "메타데이터 가져오는 중…",
     "metaFailedShort": "메타 조회 실패",
     "metaFailedTitle": "메타데이터 조회 실패",