installer-rp: add resume-on-retry and discard-on-quit for failed installs

On install failure the temp folder is now preserved instead of wiped, so
already-downloaded songs/images are skipped on the next attempt. The
error screen offers 재시도 (resume from the failed item) and 처음으로
(discard the partial download and restart). Closing the program without
retrying still wipes the partial download via window-all-closed, and an
explicit cancel also clears it.

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

src/installer-rp/preload.ts

@@ -27,6 +27,10 @@ const api = {
   cancelInstall: (): Promise<void> =>
     ipcRenderer.invoke('rp:install:cancel'),
 
+  /** 재시도하지 않고 처음으로 돌아갈 때 받아둔 임시 파일을 정리한다. */
+  discardInstall: (): Promise<void> =>
+    ipcRenderer.invoke('rp:install:discard'),
+
   /** %appdata%/.mc_custom/resourcepacks/ 폴더를 OS 파일 탐색기로 연다. */
   openResourcepackFolder: (): Promise<void> =>
     ipcRenderer.invoke('rp:finish:openFolder'),