Scaffold resource pack installer entry

Add a second Electron entry under src/installer-rp/ + installer-rp/
launched by `npm run installer:rp`. It is structurally separate from
the music quiz installer (own tsconfig, own preload, own renderer),
shares the existing styles via a relative link, and exposes a
three-step UI: pick a 음악퀴즈, run the install, then a 완료 page that
can open the resourcepacks folder or quit.

The install IPC handler currently scaffolds the flow per docs/add.md
(yt-dlp prep → music download → image normalize → zip build → place
at %appdata%/.minecraft/resourcepacks/) but the actual work is still
TODO. Cancel/cleanup of %appdata%/.mc_custom/.temp/ is wired up so
that future iterations can plug each step in without rewiring.

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

package.json

@@ -8,6 +8,7 @@
     "start": "tsc -p tsconfig.server.json && node dist/server/app.js",
     "dev:server": "tsc -p tsconfig.server.json && node dist/server/app.js",
     "installer": "tsc -p tsconfig.installer.json && electron .",
+    "installer:rp": "tsc -p tsconfig.installer-rp.json && electron dist/installer-rp/main.js",
     "dist:win": "tsc -p tsconfig.installer.json && electron-builder --win"
   },
   "dependencies": {