installer: address vanilla mods preservation, robust map rename, new app icon

Reviewer follow-ups:

1) Preserve mods/ for vanilla packs. `downloadModsFolder` now checks
   `!pack.modsFolder` BEFORE wiping — vanilla packs (no modsFolder) no
   longer clobber a user's hand-curated mods directory. Wipe still runs
   for modded packs to keep different MC versions from colliding.

2) Always rename the extracted map to `saves/<퀴즈이름>/`, regardless of
   the zip's top-level layout. The zip is now extracted into a temp
   directory under saves/, and:
   - if the temp has a single subdirectory, that subdirectory's content
     becomes the world;
   - otherwise the temp dir itself (e.g. level.dat + region/ at root) is
     the world.
   In either case, it is renamed atomically to `saves/<sanitized name>`
   (or `<name>_2` etc. if a user world collides). Marker tracks the
   final folder name for participant cleanup.

User request: replace both .exe icons.

- Added build/icon.ico (multi-size 16/32/48/64/128/256) and build/icon.png
  generated from the new music-note artwork.
- electron-builder.yml: set win.icon, nsis installer/uninstaller icons,
  buildResources=build, include build/icon.* in files for runtime use.
- New electron-builder-rp.yml + dist:win:rp script so the resourcepack
  installer also packages with the same icon.
- BrowserWindow({ icon }) wired in both installer and installer-rp main
  processes so the running window's titlebar/taskbar icon matches.

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

electron-builder.yml

@@ -2,10 +2,12 @@ appId: kr.tkrmagid.musicquiz.installer
 productName: MusicQuizInstaller
 directories:
   output: release
+  buildResources: build
 files:
   - dist/installer/**
   - dist/shared/**
   - installer/**
+  - build/icon.*
   - package.json
 # 빌드 시점의 .env 를 설치기 옆에 함께 배포(없으면 조용히 패스).
 # 패키징 후 운영자가 resources/.env 만 교체해서 도메인을 바꿀 수도 있음.
@@ -23,7 +25,10 @@ extraResources:
 win:
   target: nsis
   artifactName: ${productName}-${version}-Setup.${ext}
+  icon: build/icon.ico
 nsis:
   oneClick: false
   allowToChangeInstallationDirectory: true
   perMachine: false
+  installerIcon: build/icon.ico
+  uninstallerIcon: build/icon.ico