installer-rp: override packaged main entry to dist/installer-rp/main.js

The root package.json's `main` field points at dist/installer/main.js
because that's the default `npm run installer` entry. Without an
override, `electron-builder --config electron-builder-rp.yml` would
package the resourcepack installer with the wrong main, so the exe
would start the regular installer (or fail outright).

Add `extraMetadata.main: dist/installer-rp/main.js` so the packaged
package.json is rewritten at build time to point at the right entry.

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

electron-builder-rp.yml

@@ -1,5 +1,9 @@
 appId: kr.tkrmagid.musicquiz.installer-rp
 productName: MusicQuizResourcepackInstaller
+# 루트 package.json 의 "main" 은 메인 설치기를 가리키므로, 패키지된 앱이
+# 리소스팩 설치기를 진입점으로 쓰도록 빌드 시 main 을 덮어쓴다.
+extraMetadata:
+  main: dist/installer-rp/main.js
 directories:
   output: release
   buildResources: build