feat: add cover-image task to export cover PDF as 300 DPI PNG

Runs Ghostscript on output/cover.pdf to produce output/cover-300dpi.png
for visual QC and sharing. Also wired into the `pnpm all` pipeline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -9,8 +9,9 @@
     "build": "node scripts/build.js",
     "pdf": "node scripts/pdf.js",
     "cover": "node scripts/cover.js",
+    "cover-image": "node scripts/cover-image.js",
     "book": "pnpm build && pnpm pdf",
-    "all": "pnpm build && pnpm pdf && pnpm cover",
+    "all": "pnpm build && pnpm pdf && pnpm cover && pnpm cover-image",
     "watch": "node --watch scripts/build.js"
   },
   "pnpm": {