Set up book project: Markdown→CSS→PDF pipeline for KDP

Adds the full authoring and build toolchain for "Das Kaleidoskop der
Schlummerwelten" — all 12 story content files in Markdown, Nunjucks
HTML templates, CSS print layout, and Puppeteer-based PDF generation
targeting Amazon KDP (8.5×8.5 in, 0.125in bleed).

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

.gitignore

@@ -1 +1,5 @@
-.claude/
+.claude/
+node_modules/
+output/book.html
+# Keep the PDF in git if you want to track versions, otherwise add:
+# output/kaleidoskop.pdf