installer: clean up installer-extracted map when switching to participant

When the user installs as single (skipMap=false) and then navigates back
to choose participant (skipMap=true), the previously-extracted map files
in .mc_custom/saves/ would remain because skipMap=true only skipped the
download. The final participant install state was therefore inconsistent
with the chosen role.

Track the top-level entries that downloadMapZip extracts via a marker
file (.musicquiz-installer-map.json) inside saves/. On participant
install (skipMap=true) or before a re-download, only the entries listed
in the marker are removed, so user-created worlds are preserved.

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

locales/installer/ko-kr.json

@@ -191,6 +191,7 @@
     "labelMap": "맵",
     "skipServerZip": "서버 파일(serverPath)이 비어 있어 서버 zip 다운로드를 건너뜁니다.",
     "skipMapZip": "맵 다운로드를 건너뜁니다 (mapPath 비어 있음 또는 참가자 모드).",
+    "cleanupInstallerMap": "이전 설치에서 풀어둔 맵 {{count}}개를 정리합니다.",
     "skipModsFolder": "modsFolder가 비어 있어 모드 다운로드를 건너뜁니다.",
     "modsIndexFetch": "모드 목록 조회: {{url}}",
     "modsFolderEmpty": "/file/mods/{{folder}}/ 안에 .jar 파일이 없습니다.",