Download and normalize painting images via sharp

Add sharp dep (libvips bindings) — fastest option for the per-image
center-crop + Lanczos resize step. Pure-JS alternatives (jimp) and
spawning ffmpeg per image were both ~5-10x slower in this hot loop.

Add src/installer-rp/images.ts:
- ytIdFromUrl: extracts the video ID from watch?v=, youtu.be/, and
  /shorts|embed/ URL forms
- downloadImage: for YouTube URLs tries i.ytimg.com/vi/<id>/
  maxresdefault.jpg first, falls back to hqdefault.jpg; plain image
  URLs go through a generic HTTP/HTTPS GET that follows 302s
- normalizeToCover: center-crop to min(w,h), Lanczos resize down to
  1024x1024 when larger, never upscales, writes PNG
- coverFileName: returns cover_NN.png with zero-padded NN

Wire step 2-3 of the install handler to download + normalize each
image into <tempDir>/painting/cover_NN.png. Zip build (step 2-4)
will pick those up next.

Verified with synthetic 1200x800 and 2000x1500 buffers: small
input stays 800x800 (no upscale), large input becomes 1024x1024.

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

src/installer-rp/main.ts

@@ -12,6 +12,7 @@ import type { RpFetchedPack } from './types.js'
 import { ensureYtDlpExe } from './ytdlp.js'
 import { ensureFfmpegExe } from './ffmpeg.js'
 import { downloadMusicTrack } from './music.js'
+import { downloadImage, normalizeToCover, coverFileName } from './images.js'
 
 interface RpInstallerState {
   manifestUrl: string
@@ -187,10 +188,27 @@ ipcMain.handle('rp:install:start', async (): Promise<{ resourcepackPath: string
     }
 
     // 2-3. 사진 다운로드 + painting variant 정규화
-    sendLog(`사진 다운로드 시작 (${pack.list.images.length}장) … (TODO)`)
+    const paintingDir = path.join(tempRoot, 'painting')
+    await fsp.mkdir(paintingDir, { recursive: true })
+    sendLog(`사진 다운로드 시작 (${pack.list.images.length}장)`)
     for (let i = 0; i < pack.list.images.length; i++) {
       throwIfCancelled()
-      sendLog(`${i + 1}번 사진 다운로드 중… (TODO)`)
+      const entry = pack.list.images[i]
+      sendLog(`${i + 1}번 사진 다운로드 중…`)
+      let buf: Buffer
+      try {
+        buf = await downloadImage(entry.url)
+      } catch (err) {
+        throw new Error(`${i + 1}번 사진 다운로드 실패: ${(err as Error).message}`)
+      }
+      throwIfCancelled()
+      const outPath = path.join(paintingDir, coverFileName(i + 1))
+      try {
+        await normalizeToCover(buf, outPath)
+      } catch (err) {
+        throw new Error(`${i + 1}번 사진 정규화 실패: ${(err as Error).message}`)
+      }
+      sendLog(`${i + 1}번 사진 완료: ${path.basename(outPath)}`)
     }
 
     // 2-4. 리소스팩 zip 빌드