installer: move yt-dlp/ffmpeg under .mc_custom/installer/, clean platform-cache

- yt-dlp.exe, ffmpeg.exe now live in %appdata%/.mc_custom/installer/ so
  the .mc_custom root stays a clean Minecraft game folder. Existing
  binaries at the old location are migrated on first run.
- After a successful install, the platform-cache (downloaded fabric /
  forge / neoforge installer jars) is deleted — it's regenerable and
  was just wasting disk space.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "minecraft-music-quiz-installer",
-  "version": "0.2.1",
+  "version": "0.2.2",
   "description": "마인크래프트 음악퀴즈 간편설치기 + 관리 사이트",
   "main": "dist/installer/main.js",
   "scripts": {