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>

.gitignore

@@ -2,6 +2,7 @@
 node_modules/
 output/book.html
 output/resized/
+output/cover-300dpi.png
 output/cover.html
 output/book-meta.json
 # Interior PDF is large (~100 MB with placeholder pages) — excluded from git.