feat: Combine all core book pages into a single scrollable index.html

Replaces the iframe-based viewer with a self-contained file where each
page is a <section id="page-N">. Navigation (sidebar, prev/next, arrow
keys, URL hash) scrolls to sections instead of loading separate files.
Added build.py to regenerate index.html from the individual page files.

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

Justfile

@@ -6,6 +6,9 @@ user := "root"
 host := "goldfish.malzahn.lan"
 www-root := "/usr/share/caddy/public_html/fabula"
 
+clean:
+    rm -rf dist/*
+
 serve:
     npm run dev