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-rp.yml

@@ -0,0 +1,31 @@
+appId: kr.tkrmagid.musicquiz.installer-rp
+productName: MusicQuizResourcepackInstaller
+directories:
+  output: release
+  buildResources: build
+files:
+  - dist/installer-rp/**
+  - dist/shared/**
+  - installer-rp/**
+  - build/icon.*
+  - package.json
+# 메인 설치기와 동일하게 .env, locales 를 함께 배포.
+extraResources:
+  - from: .
+    to: .
+    filter:
+      - .env
+  - from: locales
+    to: locales
+    filter:
+      - "**/*"
+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