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>

src/installer-rp/preload.ts

@@ -0,0 +1,39 @@
+import { contextBridge, ipcRenderer } from 'electron'
+import type { RpFetchedPack } from './types.js'
+
+const api = {
+  /** manifest 와 각 음악퀴즈의 file/list/<key>.json 까지 한 번에 로드. */
+  loadPacks: (manifestUrl?: string): Promise<RpFetchedPack[]> =>
+    ipcRenderer.invoke('rp:packs:load', manifestUrl),
+  /** 음악퀴즈 키를 선택. */
+  selectPack: (packKey: string): Promise<void> =>
+    ipcRenderer.invoke('rp:packs:select', packKey),
+
+  /** 리소스팩 빌드/설치 시작. 완료 또는 취소될 때까지 resolve 되지 않을 수 있음. */
+  startInstall: (): Promise<{ resourcepackPath: string }> =>
+    ipcRenderer.invoke('rp:install:start'),
+  /** 진행 중인 설치 취소. 임시 파일 정리 후 종료. */
+  cancelInstall: (): Promise<void> =>
+    ipcRenderer.invoke('rp:install:cancel'),
+
+  /** %appdata%/.minecraft/resourcepacks/ 폴더를 OS 파일 탐색기로 연다. */
+  openResourcepackFolder: (): Promise<void> =>
+    ipcRenderer.invoke('rp:finish:openFolder'),
+  /** 프로그램 종료. */
+  quit: (): Promise<void> => ipcRenderer.invoke('rp:quit'),
+
+  /** 로그 스트림 구독. */
+  onLog: (handler: (line: string) => void): (() => void) => {
+    const listener = (_event: unknown, line: string) => handler(line)
+    ipcRenderer.on('log', listener)
+    return () => ipcRenderer.removeListener('log', listener)
+  }
+}
+
+contextBridge.exposeInMainWorld('rpInstaller', api)
+
+declare global {
+  interface Window {
+    rpInstaller: typeof api
+  }
+}