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/shared/paths.ts

@@ -32,3 +32,13 @@ export function getAppDataDir(): string {
 export function getMcCustomDir(): string {
   return path.join(getAppDataDir(), '.mc_custom')
 }
+
+/**
+ * %appdata%/.mc_custom/installer — 설치기가 자체적으로 다운로드해 사용하는
+ * 외부 바이너리(yt-dlp.exe, ffmpeg.exe 등) 보관 위치. .mc_custom 루트가
+ * 마인크래프트 게임 폴더(`mods/`, `resourcepacks/`, `saves/` 등)와 섞이지
+ * 않도록 별도 하위 폴더에 둔다.
+ */
+export function getMcCustomInstallerDir(): string {
+  return path.join(getMcCustomDir(), 'installer')
+}