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>

src/installer/main.ts

@@ -1163,6 +1163,11 @@ ipcMain.handle('client:install', async (_event, payload: ClientInstallPayload) =
   await linkMinecraftRuntimeDirs(customRoot)
 
   await updateLauncherProfile(pack.pack, customRoot)
+
+  // 설치가 끝나면 더 이상 필요 없는 platform-cache(다운받은 fabric/forge/neoforge
+  // installer jar 캐시)를 삭제한다. 다음 실행에서 다시 받으면 되고, 남겨두면
+  // 사용자 .mc_custom 폴더만 차지한다.
+  await fsp.rm(path.join(customRoot, 'platform-cache'), { recursive: true, force: true }).catch(() => {})
 })
 
 interface FabricInstallerMeta {