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>

content/00-front-matter.md

@@ -0,0 +1,7 @@
+---
+type: front-matter
+title: "Das Kaleidoskop der Schlummerwelten"
+subtitle: "Zwölf magische Geschichten für die Nacht"
+author: ""
+year: "2025"
+---