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>

src/shared/store.ts

@@ -287,7 +287,8 @@ export function normalizePackList(input: unknown): PackList {
         title: sanitizeStr(entry.title),
         artist: sanitizeStr(entry.artist),
         durationSec: sanitizeNumber(entry.durationSec),
-        aliases: sanitizeAliases(entry.aliases)
+        aliases: sanitizeAliases(entry.aliases),
+        description: sanitizeStr(entry.description)
       }))
       .filter((entry) => entry.url.length > 0),
     images: images